I was having that problem too, where over like 1 year period of time where various teeth were cracking and I was getting crowns put on them, I asked what could be causing this and the answer they gave me were that some people are just more pron to this sort of thing then others, and usually it runs in familys to, where the teeth just aren't as strong as some people.
Hope your teeth aren't still cracking...mine finally stopped.
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