Another answer could be when the ask is getting loaded or "stacked".
This happens when you look and you see 25000 shares are being sold at a certain ask, and you look a few seconds later and see 2000000 shares have been added to the same ask.
There are a few reasons this would occur.
1). Loading wall. Someone wants traders to see an impenetrable number of shares in the ask so they may dump into the bid.
Guess what. The person who put 2 million more to the ask, is sitting on the bid. He just persuaded you to sell.
2). Impatience
Doesn't matter if chart resistance is 10 ticks up. If, for the moment, traders see this ask is potentially profitable, they will stack it to exit.
3). Resistance.
If the ticker identified a new pps resistance, and traders see it will be their best exit, many traders will load an ask in hopes volume will buy them outm
