Most likely it has to do with unity embedding the mono runtime, but that's just my guess. I believe the ball is basically in AVAST's court, but they aren't particularly communicative. Kaspersky 2011 had the same sort of false positive problems with unity games in 2011, but by their next year version they had fixed that on their end and they haven't had a problem since. AVAST has been slower to move.
In the Last Federation forum, there is a stickied thread where people have discussed ways of getting around AVAST killing that game, which is basically the same sort of thing as for here. For some of them, moving the game out of the Program Files folder and then setting an exception did it. Someone just today said that if they set an exception on the entire folder for the game (in this case BionicDues) rather than just on the game executable (Bionic.exe), then that made it work for them. Other folks have had other results and just had to switch antivirus -- for a lot of them, even disabling AVAST would not actually turn off its interference.
Very frustrating. Hopefully one of those workarounds might work for you, though.