If a Task Group does stuff by itself, it should be because of queued moves. If you've set Default and/or Conditional orders, those add a new move to queue. My guess is you've set the Default order "Move to Mineral Source" to your miner, which can be logical. I've never used that, so I don't know its decision mechanics. I tend to simply micromanage my miners, since they stay at an asteroid or comet colony of my choosing for years.