There's no point in throwing out a bunch of guesses. Metal parts whose integrity has not been compromised do not break, so either the part had an original defect or was somehow stressed since that time, whether you were aware of it or not. But your title and first paragraph refer to breaking, then you say it "came off," and finally you end asking why it would break. Did it break, with a portion still attached to the bike, or did the bolt/fastener holding it on fail or come out?
Last edited by cny-bikeman; 07-09-13 at 09:35 PM.