Rear airbag exploded

Hi all my n/s airbag burst today on my 99 Disco.Went with a proper bang and scared the **** out the wife :screaming_bug_eye_f. All was ok before this happened she used to sit perfect (the disco that is)no dodgy compressor noises or owt like that sooo looking for guidance, can it burst cos it was past its sell by date or did it burst because of another issue . No idea when they were changed last.
Cheers Davy