The very likely reason is because of their small bladder. If you are holding them for 15-20 minutes or more, then he or she will need to empty their bladder. It may be he or she has a smaller bladder and it's less time, but it may be that he or she has a larger bladder and it's more time. However much time it is, that is probably the reason. You can typically tell that they are about to relieve themselves on you because that is when they get more squirmy and move more, and that's typically the time that people get nipped (I'm not saying that you'll get nipped, but it's a guinea pig's way of communicating with us, so I let you know). Hope I was able to help and welcome to the forum!
Kimmie