That amount of substrate seems a little excessive, but regardless of quantity, it still needs changed regularly. They poop and pee in the dirt and then absorb moisture and anything else in the soil back into them. So then you have nitrate and ammonia buildup as well as bacteria and fungus finding the warm, moist, loose, organic soil the perfect breeding ground.

As for why he is deciding to bury, they just like to. Especially as an adult, they can go a while between feeding and will come out when truly hungry. Being frightened or stressed could be making him want to hide. Make him feel secure by covering 3 sides or the enclosure, keeping it in a low traffic area, handling the frog as little as possible, and being quiet and gentle any time you have to do something in the enclosure.