This is not a Robinhood thing; every single brokerage does this. Robinhood is a bit unique with their instant bank deposits feature, that's something traditional banks don't generally do, but its really just a minor extension of this same idea.

Robinhood is accessible, so it has serviced the bulk of the retail investor spike that happened this week. Additionally, I expect that their net liquidity is far, far lower than a traditional bank, just given their age and diversification of business. They needed money, so they went to JP Morgan. Plenty of people also trade with JP Morgan, but (and I can't believe I'm saying this) JP Morgan actually, to some degree, knows what the fuck its doing. Robinhood doesn't; Robinhood is basically a teenager who just got a new car, wanted to show off to some friends, crashed it, and is now crying on the phone to daddy.

Which, to be fair, is exactly the same thing every bank had to do in 2007, except daddy was the US Government.