RIP
Since I can't read russian, I don't have the latest news on that, but from my perspective it seems that the automatic firefighting system was triggered in one compartment, either by accident or maybe by oversensitivity.
Since the sub was kind of overcrowded with shipyard personell, maybe there were not enough rebreathers for all in the compartment, so those who got one survived and most of those who didn't died.
Maybe the compartment would have around a dozen people with a standard crew and now it had twice as many.
Since there were injuries, it seems they opened the compartment pretty fast.
I was pretty sick of reading the usual "drunken incompetent russians" comments on german news (not naval) forums