Enemy ships sink for no apparent reason

[Zero Niner]

On patrol in the Bismarck Sea, radar picks up a convoy west of me, conveniently headed my way. So I position myself in its path, submerge and wait for it to come to me (all stop).

The convoy has 4 DDs as escorts, one each positioned in the lead, trail, port and starboard beam respectively. As they approach I keep my scope down, relying on hydrophones to keep track of their relative positions to me. The lead escort passes by, oblivious to my presence, but before it can get away far enough for me to even think about raising my scope there's an awful crashing noise and a ship sunk symbol (in red) appears on my map. Pretty soon after that the death animation of a destroyer sinking plays, and another ship sunk symbol appears, this time in grey. But I get no credit for either of these kills.

So, can someone please explain to me what happened? Did the 2 DDs collide because things get a bit screwy under time compression?