It is rather hard to dent while in the case + padded backpack or w.e it was in (too lazy to go back down and read again). Only logical explanation I can come up with is either you dropped the bag and it fell on the back side hence denting the iPad or your wife found out about your other girlfriend and kicked your iPad in the butt.
On a serious note, I am also very careful about my belongings and take great care of them. If you're worried of future cracks, a better protective case such as the Otterbox may help. Do note that I have not used it and cannot provide advice on how effective it is but I'm sure others will be able to suggest/answer those sorts of questions for you.
-- SR