market makers can make good guesses of where stop loss lie.

http://www.zealllc.com/101/zi7.htm

In the past, some traders worried that market makers would run their stops. This involved an unscrupulous market maker seeing where most stops were placed and trying to force a temporary anomaly that triggered those stops, such as selling short to engineer a fast and short decline. While still a risk today, it is quite trivial. These days many brokers keep their clients' stops in the brokers' own computers and do not transmit them to market makers at exchanges until they are triggered. So market makers cannot see them in advance. Nevertheless, since stops are usually set at fairly logical levels market makers can make good guesses of where they lie. They can probably only run them in the smallest of companies though, smaller than our average stock trade at Zeal.