i had a white oak board bow break on me a few months ago when tillering. the break looked almost indentical to yours. can't blame mine on low humidity though, summer in MN is pretty darn wet. mine broke as it was nearly finished, i was pulling it back to full draw (not the first time) and it just blew up without warning, i punched myself in the face. it was a little embarassing since i was working on it at the arcery range over my lunch break. It was so loud everyone at the range looked in my direction, all they saw was a guy rubbing his cheek and looking around like what the heck just happened, kinda funny in hindsight.
i agree with dwardo, sometimes wood just does squirrely things.