[reitanqild]

Alternative: some group in the FSB or the Russian navy thought it would be impossible to blame it on Russia (for obvious reasons), then went ahead and blew it up to be able to blame it on someone else (UK, US or NATO in general maybe?)

Somewhere along the line something went wrong (for them).

(Not saying they did it, only that think it is the least crazy of the options I have considered.)

[hackandthink]

It's a kind of nice mystery, at least nobody died (I hope so). There will be novels and movies.