Why did Fritz do this?

Was the game going into the endgame with most of the heavy pieces off the board? If so, maybe it wasn't such an illogical move. If not, then you're probably right. Although I'm not a programmer, I'm convinced that it's very difficult to make an engine replicate a novice player in a natural way. Most engines don't quite get it right. The engines that do get it mostly right can be counted on one hand with a couple of fingers left over.