There are several threads already about this, try a search and see what you come up with. About the most common cause of this is charging the watch near a fan (or where the air flow changes, wind etc) or where there are vibrations that can trick the watch into thinking it is moving.
This happened to me me the first time I charged my Ionic. I had it next to the fan, I moved the location of where I charged it and have not had this happen since. There may be other factors, but from what I've seen these seem to be the most common environmental causes. Hope this helps
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