I've seen some "runs" on Bitfinex due to Stop and Trailing-Stop orders. Since these are Market Orders, they execute at current highest bid. As highest bids are being filled, bids keep going down being "swept" by these automatic market orders. Of course this mostly happens on low volume coins, so you should be especially careful whit those.

Allow me to illustrate this with an example:

Note this is a 5 minute resolution chart.