Could be that there was no one in line behind #106.
I had something similar happen to me at a Chick-FIL-A a few years back. Lady in front paid for mine. I had cash and offered to pay for the car behind me, but they'd already paid by debit, offered to pay for the one behind that, debit also, so gave 'em the $20 I was going to use for mine and told 'em to put it toward the next cash customer.