[TorpX]

One strange thing is that the enemy will leave the rest of the convoy unprotected and go after you while while the other ships sail off.

I wouldn't consider this strange. An escorting ship really only has two ways to protect a ship:

1. Hurl itself in front of the torpedoes (not good!).
2. Attack enemy sub (much better!).

If it attacks the sub, it might sink it, and eliminate the danger, outright. Failing that, it can at least hold down the sub, and give the friendly ships time to exit the danger area. Of course, the escorts can sail on with the convoy, to "protect" it, but if they aren't going to make attacks, they are more like floating scarecrows, than any real protection.

Most of the time, the escorts would split the difference, and make counterattacks with gusto, then after some interval had passed, or the trail had gone cold, break off the hunt, and rejoin the convoy.

Remember, most USN sub attacks were one boat affairs. The "wolf pack" business didn't really get going until pretty late. Even then, it wasn't easy to coordinate an attack with multiple submarines.