Flip-flops' straps can also rub as a wearer walks, creating friction that leads to blisters.

CNN: Flip-flops present feet with a painful problem

What is suggested to be at work here is the concept of asperities - the idea that if you take two rough surfaces and rub them together, the peaks in one will couple with the troughs in the other, increasing friction.

BBC: Artificial finger tests evolutionary origin of prints