most likely they were hundreds of miles apart from one or the other and picked up a contact on sonar, classified it as a submarine and went to investigate (most likely using it as a training exercise). the ensuing drama would have involved both subs trying to stay hidden while keeping the other contact under surveillance. so know we are talking of subs only hundreds of meters apart that are both changing depth and headings continuously, and it is not that hard to see why they collided. it's not the first time either. it has happened a few times before. during the Cold War a US and Russian sub collided doing just what I described above. I think the conning tower on the Russian sub was almost torn off from tht collision.