Barking in the car

I've always had Sapphire and Lucky in separate crates in the back of the SUV. This weekend, I decided to try one larger crate and see if they'd get a long. It's a wire crate instead of two plastic crates. They behaved wonderfullly. None of the barking/growling any time the other dog bumped into one of them.... UNTIL we went through the drive thru ! I guess they could never see the person at the window before. But the minute I got to the window to pay, they started barking ! they never barked at the drive thru before !