Maybe the position at YI didn't become available until that time and he jumped on it. Was the timing bad, sure, but maybe the job was available and he felt like it was a better opportunity for farther down the road. Have you ask him? If not, don't speculate just wish him the best, and move on. It's not like he left your program for a State Title winning team!
Don't dwell on "beating Samber"....better yet, go ahead, like I said before, YI is not focused on winning for "coach". They want to win the game for themselves, the TEAM!