Apparently guinea pigs will occasionally "mark their territory" by rubbing their rears on stuff and leaving a scent behind. They may also pee in strange places, as a marker to other pigs. They usually don't do the two together, though, because they like to keep themselves clean and fluffy.
Fuzzy may have done the two a little too close to each other one time, though, as we picked him up and his tummy was all wet. Ew. Luckily he hasn't done it since.