So, how do you get two missile boats, which normally try to stay away from everyone and everything, to bump into each other?
Boomers want a quiet area, away from other 'traffic', with good access and escape routes, beneficial bathythermal conditions for evading detection and/or detecting people looking for them. Ideally, not too far from their operating base, they have to get on station without being detected. If you take all these factors into account, it's not too difficult to imagine that more than one nation, 20 odd miles apart at the nearest point, would pick the same bit of water to hide in. At their patrol speeds, they are 'holes in the water', again it comes as no surprise that they didn't hear each other before the loud clang.