No idea what happened , but a look at the submarine will clarify anything..
if the external surface is in great condition and the sub returned on its
own then it was an accident..
Which ever is the case is very dangerous for Russia nation security .
to have NATO navies navigating through the economic zones of
artic Russia.. Because between Russia main land and some northern artic islands
there are choke points ,that hostile NATO navies can use to deploy mines or
kamikazi drones that will explode when they detect another submarine of a different
sonar signature.. The same thing is true for China in the south China sea..
Any NATO warships there can deploy advanced mines and or spying equipment..
to keep track of CHina navy movements and is no wonder they very angry at
NATO navigation in the zone. So worried is CHina about NATO navigation in
South China sea ,that they are ready to invest Trillions of dollars in very large
train routes connecting Asia with Europe.. The Belt and road initiative is all about security
,to bypass any open or secret blockade that CHina predict , the Americans will do on them..
in the future.. when both nations start a cold ,semi hot war. to have an alternative to maritime trade , because CHina is too dependent on sea routes for its Economy and they Know Americans can blockade China maritime routes.. with infesting the trade routes and choke points with Isis pirates ,thousands of smart mines, that are designed to hit large ship containers.. etc...
Russia trade is not dependent much on sea routes.. they use pipelines ,that
are easy to repair , and that Europe also helps to monitor..Still the artic routes poses a
challenge to Russia own naval fleet.