About the question: "why someone mine and sell BTX now?" Unfortunatelly we can't see the whole picture. It could be that early BTX miners-investors sell because of they practically reach non-loss level. It also could be private BTX miner with better hashrate. Or smth else. Who knows...
FPGA. Though I could not imagine any nerd skilled enough waste his time on creating a bitstream for TimeTravel10, to mine and dump an illiquid altcoin that is traded solely on Cryptopia and sets ATL every day.
FPGA and professional ASIC miners don't care about "muh narrative, muh decentralization, muh technology". Well, they fare better than holders and traders that did not realize timely on what phase of the market cycle they are.
Edit: sorry, BTX is traded on other markets as well. I was thinking about another mineable Cryptopia altcoin (sole market reference).