There are several preconditions. It's not a computer message; think 1970s tech. There's a accelerometer to say you need a head-on crash and a seat mass sensor to say there's a large enough human in the seat. There are cases where the passenger bag did not deploy, for example a stuck-closed accelerometer, and then a responder knelt on that seat getting to the driver, satisfying the last condition, and causing a deployment with obvious injury.

But those conditions are met by computer / logic chip. Again find it hard to believe it accelerometer keeps broadcasting for more than 100ms after crash or so.

They contain explosives, which often don't need more than a bit of heat to go off.

A few people have pointed out that explosives can explode regardless of the message; I'd like to question, if it needs a specific message to fire them, how fragile is the system that delivers the message?

Yes, in an accident the airbag generally fires so quickly that it happens before physical damage occurs to the device firing it, but needing a specific message (over CAN bus perhaps?) makes the system much more fragile. What if there's a fender bender the week or month before, and causes light damage to the system? One bit is more robust by an order of magnitude than two, and so on.

I've seen one in a pillar fire off during training while the vehicle had no battery and the same pillar was being cut through with hydraulic cutters.