The angle between your arrow and bow arm may have been too high.

The strongest position is when your arms and shoulders form a straight line (with your shoulders low). So the closer to parallel the line of the arrow is to this, the easier it is to hold.

Now, technically you are correct. If your release is perfectly clean there should be no problem. But in reality, there will be some degree of 'plucking' going on. Your fingers can't open fast enough. So the string actually vibrates a bit from side to side. I have seen some weird follow throughs that avoid this problem.

There are very few top recurve archers that shoot without a guard.