This is happening to a lot of people during this pandemic, mainly due to all the cancellations and refunds, thus creating negative balances. Most likely got flagged by system and not reviewed by an actual person. Your statement should've indicated that you had to make a minimum payment. Yes, it's silly and makes no sense to make a minimum payment on a negative balance. But it's a known problem/issue.

Just give them a call and it should get fixed without issues. For future reference, you can either make the minimum payment, or contact the bank to either cut you a check for the negative balance or have them deposit it into your account.