windscreen affecting airbags? eh?

The glass is an integral part of the safety systems, on many vehicles the glass is used as a barrier for the airbag. It acts as a backing plate so the bag doesn't just act like a balloon when it deploys and just bounce around. If the glass isn't bonded properly it will break free and the bag becomes useless.

A few weeks ago, I saw a car (25-30 years old) get rear-ended. Apart from' the arse end crumpling, the windscreen popped off. So I guess that the glue had deteriorated badly....

Probably not the glue but what it was attached to. Most glass is bonded into the painted window channel, and rust/corrosion or failure under the paint and the paint just peels up. In my area it is the rust that causes most issues, even "sealed" connectors corrode internally from it.