@Vampire: Krolin are facists. You know, warlike and controlling of their citizens. Uber fascists considering how many "indoctrinators" they build :p
Anyhoo reasons for war/diplomatic incidences? I know you just said they don't have to be funny, but I think they can still be:
Burlust: They shutdown the chocolate factory! We demand more sweet sweet candy! (aka market based reasons, if based on TLF likes and dislikes even better. Could be also something like "Magazines you import really offend us" or "Philosophy you have is crappy")
Evucks: They are CLEARLY planning for something... We have to strike first(evucs are rather basic <_< I guess they could also be annoyed by clear stupidity of other races, think they are messing with environment because of conspiracy against them or think they are behind the monster attacks. So I guess they could attack if they get enough attacked by monsters or you terraform a lot around them or if you do something to radically change temperature. Could be also something like wind blows your pollution to their cities and they think you are doing it on purpose to poison them.)
Peltians: These guys are scary! And history has shown that we always get blown up no matter how much we beg so clearly its time to blow up them first!(yeah peltians probably wouldn't start wars :p But they are type to do pre-emptive strikes at least judging from TLF so... Yeah. They could be annoyed with your pollution ruining their crops too.)
Skylaxians: Your ambassador was really rude, prepare to die(aka honorrrrrrr
Skylaxians probably shouldn't start much wars, but in my experience from TLF they are kinda dicks xD So I think there could be few funny and non funny "HONOR" related war reasons outside of "Your people are dishonorably bullying this other races, you already beat them so stop trying to destroy them" type stuff)
Andor: Mwhaha our leader is only evil andor ever(yeah no, not gonna see andor wars. Maybe andor disputes about how you should be nicer and other party getting annoyed at them and causing war but not otherwise. Could also be involving andor parliament, but I think it was stated leadership doesn't change during this game? Anyway, I guess andor could also be so meddlesome that they sometimes accidentally cause other races to go war with each other due to trying to hold tea party or something between them)
Boarines: We don't have a isolation shield, so we have to get rid of you for being so close D:< (yeah can't really think of anything else for boarines, they are also basic. Maybe desperate demanding for cure when they are suffering from disease from incompatible temperature? xP)
Zenith: Its volcano apocalypse time! (not really seeing these guys doing wars since they don't spread much(which makes sense, growing large bodies like that probably can't be done fast) so its hard to think valid reason for them :p Maybe something that personally offends them? Maybe they are feeling really feverish due to disease and start to consider other races bacteria? xP Them misunderstanding other races demolishing their buildings and thinking they are helping other "races" by killing bacteria in their body?)
Spire: Your tv shows keep showing up in our radio, its really annoying and confusing, prepare to die (:p Seriously hard to think valid reason for these guys since they are the "We are so advanced we don't need to take care of you since you are so inferior" personality wise. Like zenith they also don't mind environment changes much since they can always survive...)
Acutians: Wind blows in such way that war is most profitable business right now./ We have acquited your resources, now to get rid of you.(aka they want resources nodes you have for themselves)/Peltian leg warmers are good in burlust market right now./All your base are belong to us make your time (Yeah acutians are easy. Capitalism/free market/greed stuff is easy inspiration for various ideas)
Yali: We are so enlightened, you so stupid, we make war now to make you smarter/Its really cramped here in this lake, so we wanna flood world to get more space, sorry you land dwellers(hard to think good or funny reason for race that thinks they are so enlightened and since pollution was already taken)
Krolin: Heil Krolin, Krolin world domination time! Lobster power!(fascists governments are rather self expository why they'd go to war <_<; )
Fenyn: Poor giant worms you keep killing for self protection, time to die!/Your terraforming doesn't involve trees hence it offends us!/Your meat market is too high!/you have too many vegetarians, thats horrific!/poor wildlife critters shouldn't live in terrarium domes!(deadly hippies is also bit too easy. )
Thoraxians: "We ignored you lesser beings when tremors on surface disturbed our hives, we ignored your annoying attempts at communication, but you have invoked our wrath. Our numbers will darken your cities, consuming you insects. Know that you brought this on yourself with your actions. Prepare for your extinction, we will erase all interspecies-thoraxian porn from your servers and our minds."
(...What? Declaring genocide over being offended by gross porn is completely valid reason(and yes I'm still stuck on how funny it is that in TLF you can make burlust like thoraxians by producing adult program channels for thoraxians) :p Its really hard to come up to funny or good reasons for bug war especially when they are supposed to be alien and apathetic to normal ethics. I think it'd be funny if queen would get talkative super villain speeches for no matter how minor reason they would decide to go for war with you xP
Either way, I think their reasons for war should be notably different from other races to highlight the part they don't really care and are alien. Maybe even something like "Thoraxians suddenly attack you without warning and suddenly stop attacking, people are scared" or "thoraxians ate a family in a restaurant and then complained it wasn't tasty". Or something similar to how bugs in ender's game didn't attack humans because they were hostile but because they didn't realize humans were individuals)
Neinzul: Umm.. Only thing that I can think of is something like Neinzul Preservation Wardens from AI War, aka they are fearing you will be exhausting all resources and their children will suffer for that so they want to stop you from doing that.
Anyway, time for race generic reasons:
Diplomatic incidences! Like "(x) and (y) ambassadors were caught in scandalicious relationship affair." or "Your ambassador learned to never play poker with acutians" or something. These could be funny, they could be tied to races, they could be generic, lot of potential on how to accidentally offend other races.
If spies and thieves are kept, could also be shenanigans on spy/thief failures and how they got caught trying to do something funny
Market reasons! Like I said earlier, for stuff like since burlust likes sweet a lot, they want all of them or think you aren't producing enough for their tastes. Or even stuff like evucs and burlust going to war because evucs hate sweets.
Anyway, sorry if these are too crappy or try too hard to be funny, I'm not really funny person :p I can write more normal ones if you give more feedback on what type of stuff you want, but I understand if you don't want my suggestions since I go for over the top easily and I do realize most of these are fluff reasons and not gameplay xD Of course it was just pointed out you don't need suggestions, but giving them is fun anyway, hence why I keep writing these walls of text of bad ideas xP