Strange Automine behaviour

[Futabamaster]

I have a set of ore miners scattered around sectors and one gas miner, they actually do work, mining their business and trading goods on stations for my profit, but! Every time they full their cargo up, captain decides to drop some of cargo in space, no threats or other obvious things happens on those occasions. He just drops 5-10% of cargo, i think, as a container, goes around for a little bit and then heads to station to sell remaining. Am i missing something?