Lack of experience and system knowledge. Perhaps there is a problem with supervision, whether it be lack there of or pressure from same to just produce cost rather than quality.
All it takes is to read the Maintenance Manual. It's not rocket science.
Safety the bottle, disconnect the pressure line and it wont go off.
If it does, look at everything. If the supervision doesn't stress to the individual after an incident like that to look at the chain etc then the supervision is too busy to protect the`travelling public.