Ground wire sparking
Either the junction box has power on it because of some loose hot wire or the ground itself has power on it because of a loose wire somewhere along the circuit. 

Most likely though, since the ground doesn't blow the breaker on it's own, the junction box is powered somehow. In one way or another, power is feeding through that box. It could be from the sheathing of the BX or any stripped or loose hot wire touching any of the system. Go back and double check all connections, something isn't right. Be safe.