Yes, it could be behavioral in the fact that cats like routine. So changes in his routine might upset him.

My first suggestion, (after keeping his routine steady) would be a 2nd litter box (if you only have 1). Some cats choose to pee and poop in separate areas and he may just decide he'd like to do this.

My 2nd suggestion is a vet visit to rule out illness. If he is not feeling up to par, this could cause unwanted behavior. Sometimes a cat will associate the litter box with the pain they might be experiencing.