MrtzaI'd be willing to bet they do it on purpose!
If they see MD or another competitor offering a watch they have stock in, rather than meet the competitors price, they may opt to simply list theirs as out-of-stock for a week or so. The part about reducing their at the same time is just more "marketing"--I could be wrong, but it would be interesting to go back and check Joma's price and stock status after MD finishes this drop ;- )