It could come from a lot of things. Heat damage, relaxer over or underprocessing, rough handling (not using the right brush or comb, not combing/brushing/detangling properly, brushing wet hair, etc.), etc. Also, once the ends are split, they need to be cut. If not, they can continue to split up the strand and even more hair will need to be cut.
If you keep having this problem, I would suggest you get a good trim, that way you'll have a clean slate to work with. And then work hard at maintaining those ends from there.