What Xara actully does is what it is intended to do... It's the configuration of your tablet at fault. The driver of your pressure sensitive tablet is recording that the level has returned to a value that is equivalent of "no mouse down". Since this happens just around that 2 pix click-width Xara beleives that you have zoomed in a square of, say, 3 pix and zooms to its maximum level (25900% ?)

Best thing you can do is to play with your setting of the driver and especially the levels of pressure sensitivity. Go for the "soft"options in Wacom's setup and just see if that fixes the problem.

Best of Luck@!