Sucks to hear man. The combination of a few things prob led to this spike.
removal of gravel. While most of your BB will be in the filters, the gravel does contain a bit.
water change. I would imagine if you have conister filters you turn them off during a wter change. Depending on how long they are off for, the BB could have started to die off.
Addition of fish. You have been adding medium to large size fish to your tank pretty rapidly and you BB may not have had time to catch up with the extra ammonia that was being produced.
What filtration is on the tank?