Allergies can be a bit funny, you don't necessarily have a bad reaction the FIRST time you are exposed to an "allergen" but generally, it comes the SECOND and further times you are exposed.
The first time, the body is just realizing that it is allergic and prepares a response for the next "attack" of the allergen.
So it is very plausible that this person had been stung at an earlier time with little reaction and then the next time he was stung it cause anaphylactic shock.
Some people can react the first time they come in contact with an allergen but majoritarily, the worst reactions come from subsequent exposures.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!