[Old Teufel Hunden]

And the Navy also insinuated that a couple of the guys were gay and it was some sort of jilted lovers quarrel thing and he wanted revenge on his former lover or something like that. It was real sickening stuff that some brass was trying to perpetrate to save their butts. Unless you have some good proof, you shouldn’t be smearing a guy who died while serving his country.

[Eric in the Ozarks]

It was more than likely caused by a spark that ignited one or more of the powder bags before the breech was closed.
I believe the bags in use in the 1980s were 40 years old. Nothing good happens to aging gunpowder or explosives.