Zeke my only thought is the relay is being held on to long either mechanical or the coil hold voltage is being held high to long.
The other thing that is disturbing in you see a flash when the relay opens...
IS your scope dual trace, might be interesting to note the time between the pop and the coil voltage.
The other thought is is it a teeny pop or are you really pushing the woofers ?
I assume this is just 2 speakers hooked up to a stereo, no external sub stuff or any external processor ?
( yes reaching for straws here )
This is a bit of a stumper....