Speaker Pop

Hey all- On my E34 I have an interesting phenomenon... When I turn my car off, the front left speaker in the foot well "pops". This happens regardless of whether the radio is on, or off (always turn the radio off when I park), or even when it hasn't even been turned on during the whole drive! Kind of driving me crazy. It started happening after I replaced the plastic dead pedal trim piece. I kept the same speaker, same wires, etc.
I took out the trim and re-installed everything, making sure the wires were tight and connections were good, but still the same problem... any ideas on what it could be??

Well, almost two years later, and three different speakers later, no more speaker pop when I turn off the car!! It must have been both speakers I installed before this latest one today were no good and caused the speaker to pop. It's only been one day though, hopefully it holds up!