I am an IT guy that worked at NYSE for a few years. Not a trader, just an IT guy. There are algo servers that trade based on algorithms, no human intervention. Bad algorithms can cause a selloff completely by accident. There was at least 1 instance of that while I was working there. Exchanges halt trading to stop an economic collapse. Either way the system is definitely rigged for the big banks but I don't think halting is really a tool they use often.