TraderWJ,
My sell stop is 30, and everybody, especially market makers know that there are huge number of stops located in 30. They begin to drop down their bids without reason. Once the bid hit 30, many sell stops are elected.
Not an uncommon scenario, and I discovered very early in my trading to never set a sell-stop at any whole dollar amount, and especially at a "0" dollar amount (i.e., 10, 20, 30, etc.) In the days of fractions I generally set at 7/8 and frequently 13/16 (in case they were running stops down 1/8 below whole numbers). With decimals the same type of safety rule applies, and .88 or .89 is much safer than .90 or especially .95.