1) There is a racial research, that means every time you lose a ship/structure, bounty gets put on the attacker.
2) If it isn't that, then it is probably the AI trying to coax other AI/players into attacking you.
3) If it is only you and the AI left, it is probably the AI abusing another racial research which boosts the amount of bounty they put on you, but still only costing the original price. Then killing your ships to claim the bounty back, and thus creating an income.
4) Otherwise, I have no idea