I don't think it has anything to do with overwatering. If it were overwatering then the signs would be clearly visible on the plant.
It is the roots instincts to spread and yes for the reason to find nutrients, but once you bring a plant indoors and/or confine the root system to a little pot, the roots will spread no matter how much feed you give them, this is their instinct.
It's perfectly normal, that when you water that the top layer of soil will sink and reveal roots. Just cover them over zed', no problem.
BTW, i did admire your pic's earlier and I'm amazed at how well you've done with it so far. You seem to have a green thumb.