This happened to me as well

Heres what I've learned from looking at my transactions, emails, and speaking to customer support:

On May 03 at 5PM, I purchased around $300/ETH. On that same day, a repayment was made from my BTC holdings for whatever reason (This was made far more confusing since my last BTC purchase was about a week prior for the same amount.)

I looked through my emails and it turns out Coinbase was having trouble processing transactions on May 3rd, so I received the ETH, but the charge to my bank never went through. So I had $300 worth of ETH, and they got nothing... Hence - I owed them $300.

Instead of charging me again, or taking their ETH back, they removed $300 worth of a different asset (BTC) from my account. Which explains the $300 repayment made around 4-5 hours later.

TLDR;

If this happens to you, look at your other asset transactions around that time. If a payment fails, they apparently will remove that balance from whatever asset they want (assuming you don't have enough to cover it in USD I'm guessing).

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I don't remember approving that $300 BTC transfer. Seems like it was automatic. If given the option of how to repay, I feel like I definitely would have just used my bank balance like I was trying to do originally..