1) Large order sent to market
2) Exchanges with a serious lack of liquidity
3) stop loss orders making things worse.
Everyone has their personal pet peeves, mine is stop loss orders. It's one of the 3 things that amateurs tend to use with out any understanding of markets. The other two being use of margin, probably doesn't need any explanation, and trading currencies/currency pairs.
In today's markets, stop loss orders are like market orders........ 99.99% of the time only people who don't know what they are doing use them.
https://news.ycombinator.com/item?id=13847775#13848698