Can you swear by God, that you didn't put your cursor (maybe accidentally) into the tags' text field and refocusing afterwards? Then you have triggered the automatic reorder. Which means, you have re-organised the tags.
This is basically by design, as tags are now automatically reordered from left to right in order of popularity. So if you edit a post, and the tags happen to be in the wrong popularity order, that will trigger a retag on your behalf.
The good news is that it should be relatively rare.