What kind of mains adapter are you using? Could be that there's insufficient power getting to the charging dock, and the flutuating drops in power will be waking the watch up as it thinks its been removed then replaced. If you've another USB adapter (ideally an Adaptive Fast Charger) then I'd try that.
The above is just an idea. I don't use the supplied charger but charge mine on a Duo pad. Takes a couple of hours to go from a few % to full.