Currently, tags are ordered by count. The most used tag appears first on the list, the least used tags appear at the end of the list. The list doesn't change dynamically, it only changes when you refresh the Tag Panel.
Try this (assuming there's no tags): tag an object with 'tag1'. 'tag1' appears first on the Tag Panel. Tag an object with 'tag2'. 'tag2' is added to the end of the Tag Panel. Tag another object with 'tag2' and refresh the Tag Panel. Now 'tag2' appears at the top of the Tag Panel.
I think that ordering tags by how often they are used can be very useful, but also ordering by name has its use too. The ideal case would be to allow changing the order of the tag panel with a toolbar button.