My understanding of SRS systems is that the power source going to the squib on an airbag system goes through a "safing sensor" which is a switch that closes at a pre set deceleration rate. Therefore the vehicle must be decelerating at a certain rate along with an impact sensor signal to initiate the airbag to trigger.

Probably doesn't explain why the side airbag went off, but it should only have triggered if both an impact sensor and the safing sensor were activated at the same time.

Tommy