Male guinea pigs (and I believe sows too but I am not sure) have glands in their anal sacks for marking their territory. When they are showing off who is boss they will do what this male is doing. You may or may not have noticed but there is usually a strong smell that comes from them doing this. A lot of times it will look like there is something white on their bums.
This is also what makes anal sack cleaning so much fun *NOT*. Males will rub this on the bottom of the cage. Hay, hair and other random bits of junk will get in the sack, which is why I always suggest cleaning their sacks every month to every few months depending on how messy their sacks are. The only real way to tell how messy they are is to clean them, if you find they are not very messy, wait a month longer and try. Each male is different.
Reko my grey pig needs his done almost monthly because for what ever reason his sack is always dirty. Mojo on the other hand needs it done every 3-4 months. I have found that neutered males don't get as "gross" down there as unaltered males. Sows hardly ever need it theirs cleaned, they may be because its a male thing, I personally do not know.
