Hmmm, this reminds me of what my rabbits used to do. They were sold to me as two females but turned out to be two males and we had to seperate them. They had a hutch each at either end of the pen and we could only let them out one at a time. One of them would run past the other and as it passed it would leap into the air and flick it's rear end. This was a bit cute a first but as they grew older the leap and flick was accompanied by a jet of urine. Eventually both rabbits (albino dwarfs) became a permanent dirty reddish brown colour. I guess this was some sort of territory marking. I know these were rabbits and you have guinea pigs, but your question reminded me of it. It may be something similar.