Originally Posted by AgRider

I don't know if it is true for 370z's, but many cars have air pressure sensors in the doors to improve side-impact performance when used in conjunction with acceleration monitors. These sensors measure pressure changes in the door cavity itself.

Making Sense of Automotive Pressure Sensors | Sensors

So, I guess it is theoretically possible that an aftermarket speaker could be disturbing your airbag system. If the back of the speaker is not sealed, maybe using a speaker baffle might help? IDK.... maybe worth a try, as I don't think they are too expensive.