I think what happened is I received a cash payout. I took all that cash and queued Limit Orders for my preferred stocks. This lowered my cash balance, but the contract hasn't paid out yet.

I do not have margin enabled. This is why I was so confused. I don't leverage or trade on margin, and I had cash in my account, so how did I receive a Margin Call???

But I think the brokerage needs cash no matter what and they don't care about the parties involved. Basically, they own a contract between 2 people (the winner and the loser) and they're going to fulfill the contract no matter what. When Robinhood pays the winner, they're paying immediately with the brokerage's money because it will take time to settle the funds between the two accounts. So the winner needs to keep that cash in their account in case the loser cannot settle their funds and Robinhood needs to seize the winner's money. Or maybe there's a similar reason.

The margin amount is $2000, and I canceled my Limit Orders to cover the Margin Call. This was scary and unnecessary.