My cat start sitting in the litter box. why?

Well. for a week now, my mom has been here with her dog, but I am not sure if the dog is the reason for this, but, we ended up putting another litter box and their food in our bedroom because my 2 cats did not want to come out of the bedroom at all for the first few days...Now, all the sudden, my older cat Just sits in it.I don't know if he is guarding it, or if he is just sitting in it for the heck of it? What do u all think? He has never done this before.