Transport Madness or a bug?
 

So I was on patrol of Honshu running on the surface at night when radar detected two ships moving fast towards me. I set myself up in what I thought was prefect attack position. I spotted two transports (Huge European Liner and a Large Old) travelling at 17-19 knots. I knew they would pass close in front but it was to close and they spotted me. I was shocked to see them slow down to 4 knots and turn away. At that speed I was able to submerge, reposition and easily sink them both.

I have witnessed this quite a few times from fast groups especially transports. Has anyone else seen this behavior?

Yes, it has been noted and reported in the bug thread. It happens quite often, although it usually happens after a torpedo is fired in my experience.

I had this exact thing happen with the same ships last night. I was chasing them at flank speed, and it seems that once I got within a certain range, they did the slow and turn thing. I actually sank them both with the deck gun! :o I shot up the large liner and the other just sat there until I got around to it.