After I originally posted about this, I did a Google search and saw that this kind of thing happens quite a bit. A friend of mine at work said that his cat for a while used to hump the floor of his house.

As far as something like a UTI goes, I looked the symptoms up last night, and there are no symptoms of something like that. He is regularly using the litterboxes (sharing them both with Smokey) with no increase/decrease in usage, difficulty in going, or blood or anything in his urine or stool.

Last night, I came home from work, and he began kneading the cushion with no humping, so I'm thinking he's just getting a bit of overly pleasured or something at times with it. It's no problem for me. I was just surprised that he was doing it since he was fixed. If he begins to do it regularly, I'll just put the cushion away for when my family visits with my 5-year old nephew since I don't want to explain to him what Casey is doing to the cushion.