The owner has passed away
The estate owns the vehicle until it transfers, and whoever signs needs the authority to sign. An executor, an administrator, or in many states a small-estate affidavit that skips probate under a value limit.
A surviving spouse often has a simpler route. The form names and the limits differ by state.
A signature from somebody without that authority does not transfer the vehicle — and it surfaces at the counter, after the money has moved.