Robinhood sells their order flow to Citadel.

Citadel is financing Melvin Capital's GME short. The financing terms are private but whether they just lent them funds to cover the margin calls or actually hold their short positions until the storm passes for a fee, it is absolutely in their interest to prevent GME from continuing to skyrocket. Drawing a collusion between Citadel and RH isn't that far-fetched but I obviously have no insider knowledge of this.

Most brokerages sell their order flow and Citadel trades 25% of the equities market in the US, from memory. Schwab, which also sells order flow to Citadel, requires 200% margin to go long GME which makes no sense since at most, you can lose 100% of your bet.

This is not what free markets are supposed to be like.