You may not send market order when the stock is in a halt. So you must have sent it before the stock halted. Hence, I'm assuming your large order either was front run or spooked the market and itself caused the halt.
I use Fidelity, so I seriously doubt it was front run. I also don't see how an order of 500 shares would spook anything.
Oh sorry, this thread crossed in my head with another one where guy is sending large orders to IB. Buy in any case, this appears to be a very thin stock, 500 shares market order can sweep several levels of the book causing the halt. For details, would need to go to Time and Sales. Don't send market orders for stocks that are thin!
Fidelity is sending a great deal of the orders to predatory HFTs - the penny jumpers. The only way around this is to send orders to the IEX, & Fidelity does not have DMA to the IEX. Citadel, Virtu, & G1 are front runners. https://finance.yahoo.com/news/fidelity-scottrade-deny-clients-anti-155357950.html https://capitalmarkets.fidelity.com/app/literature/item/9890182.html
I don't agree here. Payment For Order Flow doesn't mean that the HFT firms can do whatever they want with the order. Definitely they would not engage in the ILLEGAL front running. BTW, my understanding is that most HFT wouldn't bother with thin stocks anyway. They most likely simply forward the order to the exchange or act as a counter party.