It didn't happen to me but it happened to a friend.

There has been a Coinbase phishing campaign going on over the last few weeks. Did you recently get an email reporting that an amount of bitcoins were transferred out of your coinbase account?

It was a phishing email.

If you clicked on the link in the email and logged into Coinbase, then you gave the scammer access to your account. If you have API access enabled, then the scammer can bypass 2FA and log in when you aren't around.