I wonder if he was trying to clear the line. I did not blue font that because it's only a half jest. I have seen people feign illness to try and intimidate others out of their way. And then you have the people who will take it to the next level by faking an illness to try and scare the cops out of arresting them.
edit: I see you said this was middle of an aisle, so not the checkout I presume. I think the same thing could apply though. There are people who will go out in public when sick, which is stupid. And then there are people who try to use the threat of contagion to scare other people around them.