Click and drag will produce this behaviour.
Could you be dragging the tag while you are clicking on it?
That could be you moving your hand or your mouse being a bit flaky and reporting a drag that you didn't do?
Just to cross suspects off the list:
1 - try being ultra careful in your clicking and see if you can reproduce the behaviour when your are 100% certain you did not do a click and drag with your hand
2 - Try a known reliable mouse (or, better, another input device - trackpad, tablet, whatever)
If you can prove it is not some real instance of click and drag or an artefact of a glitchy input device then you are getting closer to someone being able to identify that the problem does lie with ST.
I have never experienced the behaviour you are reporting and it seems others are not flocking to report they are having the same issue on Win7 so it is possible (although very far from certain) that it might be something you or your hardware is doing.