Sometimes, I noticed that when I'm in hand to hand combat, a small explosion happens when they die. When is this scenario triggered? And how much damage does it do?
I believe what you are referring to is a new property of the Zoltan. They now explode on death, dealing some damage to nearby hostile forces (not systems/hull). I am not yet sure how much damage they deal, but I will determine that soon(-ish) and update this answer.
According to this wiki, Zoltan explosions harm hostiles in an entire room, not just who killed them. The amount is still to be determined.
If you have noticed it with other species, let me know because that would be something different that I am not aware of.