Why does dough break when kneading, and how to prevent/ameliorate it?
It sounds like it didn't have enough liquid in it. Assuming we are talking about a normal wheat-flour bread dough, I can't think of anything else that would lead to cracking and breaking. Properly hydrated bread dough should be quite moist and elastic, and the only way you can "break" it is to stretch it hard and fast with a pulling motion.

Fixing this situation is hard. You can try to knead more water into it, but it has a tendency to just slip off the surface. Try spreading it out as thin as possible, spraying or rubbing on a little water, folding and kneading and repeating the process.