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 15 damage to nearby hostile forces (not systems/hull).
According to this wiki, Zoltan explosions harm hostiles in an entire room, not just who killed them.
If you have noticed it with other species, let me know because that would be something different that I am not aware of.