They may be looking for food. Mine will come out of hiding and roam the tank looking for food. In my case, because I have so many, and my tank is well established, I don't purposely feed them. They eat dead plant leaves, left over fish food, and algae.
They are nocturnal, so what I would do is turn off the tank light and put in a bit of extra fish food, or a piece of algae wafer for them, leave the food in for the next 24 hours then remove any uneaten food. Hopefully by the next day they will have had enough food and be willing to burrow under the substrate. Aim to feed them once or twice a week but no more unless you want an MTS population explosion.