Does anyone know why this might have happened?
Check your account and validate if someone else was doing things in your absence... if so, then you need to change keys, because you might have leaked them somehow.
EDIT: Looking at your account, you don't have any unfollows for at least the last 5 months! So maybe it was a confusion?
By the way... change your recovery account from steem... if you get hacked, they will not allow you to recover your account.