For me it was because I was living in an apartment where the heat register was directly beneath the toilet paper dispenser, and so in wintertime when the furnace was running, if the TP was over it would flap around like one of the wacky inflatable tube guys you see at car dealerships. I started putting it under at that point and the habit just stuck.

But yeah, cats is absolutely the most common reason. Toddlers as well…dumb ones anyway. The smart ones figure it out regardless.