Guinea pig jumps away when lightly touched?

I have two guinea pigs (both 2 years old, siblings, male, unneutered). They're both very friendly and come onto my lap when I sit in their enclosure and have food. So everything's going pretty well. They're healthy and haven't had anything serious happen so far. What I've noticed is that if I touch their rear ends or their sides even just lightly, they jump away and suddenly twist their heads (sounds bad, but it's not). Does anyone have any idea why this is happening? Is it the same with yours? Is this normal for guinea pigs? Thanks in advance. I'm really looking forward to your answers 🙂