Enemy ships sink for no apparent reason
 

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?

Depends....

I assume running TMOwTW? Most likely a collision. Possibly a air strike ? Depending on date/time could have been Allied bombers in area.

In mods with AI submarines such as TMO update, FOTRS, possible attacked by other submarines, although rare. Other day while testing in the campaign, I was approaching a merchant when it suddenly exploded, a check revealed a submerged submarine on the opposite side. Were the seas heavy? I recall in older versions of TMO heavy seas would sometimes wreck the ships, waves and hull strengths had to be adjusted.

I have seen sometimes in the confusion after a torpedo attack on a convoy, destroyers crossing through the formation, only to be ramed, or to ram - a mechant, resulting in one or both of them to sink.

I guess this could be possible for a convoy zigzagging as well?

Running GFO.
This happened before I attacked - I was submerged, waiting for the lead escort to pass and for the first row of merchants to get in range.