Whatever answers you think you'll find here, will be at best, assumptions. Not even a watchmaker can accurately tell you the exact cause without taking a peek inside.
Best to just send it in for a warranty repair.
Having said this, here's my assumption
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A friend of mine had the same thing happen to his Explorer II and after opening it up, the repairman said something came loose. He fixed it back in 2 mins & everything was fine again. Apparently my friend may have bumped his watch at a certain point to cause that.