psyc_ Sure, classical conditioning. Some people are afraid of snakes; just thinking about them, and their involuntary reacts to their image of one.
When your friend steps into the shower , what does he think about?

. This is a version of PSTD; a milder version is woman who are afraid of spiders and/or mice.
This explaination is not the solution or cure.
A stimulus-response has been associated/conditioned to 'something' and the memory of is enough to replicate the emotions (not the event itself).
The explosion was a one time event, but the memory persists. A effective way to detach/sever the connection is by 'Tapping-EFT" ( Search web; youtube&facebook have demos).