Slave Miner Killed By Seemingly Nothing
The only thing that could kill your slave miner and make the slaves neutral is one of your own slaves, as you can see the one that destroyed it had received a promotion as well. So I would say that slave somehow got stuck maybe and started beating it with its shovel till it got destroyed. Other players can not do it since than the slaves color would be the one that destroyed the miner, if a neutral entity e.g. an animal would attack it than the miner would kill it with ease.
So yeah it was your own slave.