Yesterdays Trading, page-6

High Frequency traders (HFT) use a strategy called "sniffing" (in theory it's illegal but almost impossible to prove) to trip stop losses. A description of the technique is here. The general rule is not to insert automatic stop loss trips on illiquid shares like SLX as these algorithms look at sales volumes and attempt to cause an "avalanche' effect to buy cheap shares. They often pick opportune moments an example saw was HFT triggering an avalanche to coincide with the finish stretch of the Melbourne cup; the price movement (downwards) was enormous for just a few minutes; they sold and then bought back more in just a few minutes. The advice I have received in to keep your stop losses in your head (don't put them reveal them to your broker) and sell when you want to aviod being manipulated.