What allows a bank to repossess someones car?

Things may differ state to state, but in MI the lien holder basically owns the vehicle. I have the title to my truck, but it's got a big stamp on it from the credit union. The right to repossess the vehicle is in the paperwork that was signed at the closing of the loan, which also lists the vehicle as collateral for the money that was borrowed. If you can't pay the money back they have the right to take the vehicle because it's worth roughly the same amount.