Apparently the reason Robinhood halted purchases of $GME was because they received a middle of the night call from the National Securities Clearing Corporation (NSCC) demanding $3 Billion immediately[1]. The NSCC refused to explain exactly how the $3 Billion was calculated, and they eventually settled on a much lower number in exchange for RH (and other brokerages) agreeing to halt purchases of specific stocks (the stocks that were shorted by the hedge funds).

After some research into the NSCC, looks like they're a very opaque & private company ran by a bunch of former hedge fund guys[2]. Also, their board of directors appears to include some active hedge fund guys. Considering this is all it took to block millions of retail traders, I'd say we have a very serious situation that needs to be investigated. Especially considering the trades involved were extremely time sensitive (e.g. put options set to expire worthless unless the stock dropped dramatically, stalled purchase momentum, etc).

[1] https://www.youtube.com/watch?v=2M7X2dsW_Xw&t=5m25s [2] https://www.bloomberg.com/profile/company/15236Z:US