I have found this to be an older male guinea pig issue, but I've heard it can involve younger guinea pigs as well. i don't make a regular practice of cleaning them out, but this video causes me to think I should keep a closer eye on things down there for my current boar.
I have been breeding rabbits for a quite a few years. I thoroughly enjoy them as animals and think they make great pets. I also like to take some of them to rabbit shows to see how they measure up to the standards.