"Can't understand why someone would sell 500 shares at that price?"
To block trading, a part of price control.
With some brokerages you have to place an order as "All or None" to be charged one commission price, otherwise you get charged for each block that your order was filled with. That acts as a disincentive to price sensitive clients such as me.

( p.s. I trade through 3 different brokerages, because each has it's pluses and minuses.)