Have you contacted the installer? They'd be the best ones to address this. Hopefully, you're not out of warranty. As to the cause of the crack, it can vary. I'd have to question your last statement. If you open a cabinet door and look up towards your countertop, you should get some idea of how it's attached to the cabinets. I can't imagine the granite being glued directly to the cabinet tops but I have seen stranger.
Maybe someone else has some insight. If not, you should have a pro take a look, preferably the installer. (Did they make that comment over the phone w/o even coming over?! Amazing.)