Internal pressure can be fine too, but since its clear that there's a need for a gameplay mechanic that paces out the game, making those internal and external pressures explicit triggers means that there are more ways the player can interact with the solar system strategically, and it means that their short-term goals can be more varied.
Right now there are a few reasons you might want two races whom you intend to join the Federation to go to war with eachother, but its often sort of a gimmick to get access to tech without taking influence hits or to build influence with one of the races by defending them or to earn credits - very player-centered, basically. But if certain races needed to see the horrors of war to realize that they needed to federate, it might make sense to e.g. have the Boarines go to war with the Peltians to scare the Peltians into the federation. Then once its done, you have to fix the damage you caused if you also want the Boarines to be in your Federation. So it makes things more nuanced and strategically deep, hopefully.
I guess the question would be, aside from the obvious one (scare the Peltians in), are there reasonable internal/external pressures to trigger the various other races that wouldn't just be trivial - in the sense of happening too readily on their own without player intervention?
Here's another one: maybe the Andors don't like seeing other planets in turmoil, so the trigger for them is the formation of a pirate nation/sufficient amounts of piracy, seeing at least two planets with Public Order less than -100, or witnessing an atrocity (Evucks blowing up their planet, Acutians blowing up someone else's). Those are all relatively nontrivial.