** 1. does the water come from a leak underneath the unit? or behind the unit? **
I'm afraid you are the only person in a position to find that out.
** 2. what could cause the leak? **
Water could come from the fridge's water system (eg. icemaker or dispenser or connections, etc. - if used on whatever model of GE fridge you have) or from the defrost system.
** I think GE should replace this unit FOR FREE **
I wouldn't suggest you hold your breath... even if it was still under warranty (see the following link).