Account with username suddenly switched to Google login

I implement Facebook, Google, and Twitter log-in on my Meteor app, as well as allowing users to sign in manually with a username.

A couple of my users abruptly stopped being able to login with their username. One user had a username before, but today they couldn’t log in with their username anymore.

I dug through the user table and found that the user now had a Google log-in method but no username, and I confirmed in the backup that previously the user with the same id had a username but no Google log-in method.

How on earth did that happen? I don’t provide any mechanism for switching the account to a different log-in method, and the user is not aware of anything that they did that would trigger this.