I wonder if it had something to do with wearing two shirts. Not sure what the connection is but since it was important enough to include in the story there must have something meaningful in regards to those shirts don't you think?
Maybe Bob suddenly felt overly warm and uncomfortable and decided he should go back home and remove one of those shirts?
Maybe Bob noticed someone staring at his shirts, also noticed the person was armed and then suddenly got a hunch that possibly his shirts could become an issue and he remembered he had left his legal firearm at home. He may have went to retrieve his arm just in case he came in contact with someone who was interested in relieving him of one or more of his shirts?
I've got a pair of pants like that. A new pair too wouldn't you know? I've no idea how they got stained. I only know this. They aren't taco stains.
It could be that he meant to put a less valuable shirt on top just in case he dripped taco sauce onto it. While in line he suddenly realizes that the outside shirt was one of his most favorite of his collection so he decided to go somewhere more appropriate to take his shirts off and rearrange them in a more favorable way so when he eats, he doesn't get a stain on the one he likes more?