Clue by Clue: Drive Through Drama Solution
The man walked to the restaurant, so he must have walked through the drive-through lane. This is very dangerous. Other customers are in cars. A car could hit him in the drive-in lane. He could also slow the restaurant down.
This was inspired by a number of real-life examples, including an incident in Scotland where a man too drunk to drive tried to make vroom vroom noises in order to fool the drive thru servers.
If you're curious why you generally can't walk through a drive through, here's McDonald's answer