More often than not I don't even understand what kills them. Sometimes fighters will survive a huge encounter with tens of ships on each side, but sometimes you send them to help in a 1v1 situation, and half of them will die without them even being focused. I followed them very closely, and they would just pop almost at random. Splash damage? Stray projectiles?
I had a situation where fighters died even without anyone in the system. I had two flights of fighters salvaging a massive wreck, while also salvaging it with turrets myself, and I had a fighter die. I am thinking that either one fighter shot another, or maybe my salvager hit it.
It really is hilarious at times.