They feel the need to be important among other fans of a popular thing. Can't just be one of the masses who like the thing, you need to be "that one guy." That one who was so dedicated that they waited line for the Szechuan sauce, drove all the way to New Mexico to throw a pizza onto someone's roof, the one who backed the right character ship, etc.
I guess it depends on the motive. To me, all of those things could just be fun things to do and you want to share that fun you had with others (absent the one that involves trespassing and making a mess on someone's private property, anyway).
But i guess if your sole purpose for doing them is to prove your own superiority, that's what makes it toxic.