This happens when your account also has a password set, most commonly because you originally registered with email + password and only later started using "Sign in with Google" as a day-to-day shortcut. Both methods stay attached to the same single account (see "Signing in with Google vs. email and password"), the password doesn't disappear just because you stopped typing it.
Why this isn't a bug
Some sensitive actions, most notably deleting your account, deliberately ask for that original password as an extra confirmation step. Google sign-in itself never involves a password, so it can't serve as that same kind of proof-of-presence, the password check is a genuinely separate, additional safeguard specifically for high-impact actions.
What to do
If you don't remember that password, use Forgot Password to set a new one. This works identically no matter which method you normally use to actually sign in, once set, you can use it for exactly the confirmation step that's asking for it.