They called to either confirm your purchase and financing, tell you to bring the car back to the dealer, or increase your APR %. If there was no finalization from the underwriters the moment you took the car home, there's no way to guarantee the car is yours until you make your first payment.
This happened to me when I bought my last car. The dealer will get you hyped about the car, let you take it home with a "pre-approved" contract that has not been looked at by the financing company/underwriter. So I had my car for about a week and was called by the finance manager at the dealer I bought the car from. He informed me he had bad news. Which ended up not being so bad. They had to increase the APR by 1.2% from 3.5% "pre-approval APR". So I had the option to go back to the dealer, pick up my down payment and return the car or just take the 1.2% more. Luckily I was dealing with an ethical dealer, rare right? They ate the 1.2% in the cost of the car in order to compensate for the APR % loss I had. Long in the short, it really just depends on the dealer you buy your car from. I've been to and heard about really sketchy dealers. There are dealers in my area that will sell you a car and loop you into a 72 month loan at 24% APR.
The most effective thing to do to get a lower APR is to ask what the APR would be if you shorten the auto loan term. Say if you had an approval for 60 months at 10%, ask what the APR will be for a 48 month loan. You'll be pleasantly surprised, that extra 12 months takes a toll (in a good way) on that APR, bring it down to maybe 5% APR. Of course, this all depends on your credit situation.
