Oh drat -- I forgot about tone, and universes with blurry edges. Although that's a bit more about the "how" of crossovers than the "why" of them, maybe.

Crossovers are a very knee-jerk thing for me as well, which is why I've had trouble thinking about them. They're so obvious to me that the whole question of what it is I can do or am doing with them is extremely difficult to frame. It's a bit like asking what I'm doing when I walk down the street -- you know, I'm just walking down the street. But the "aha!" moment -- the contrast or comparison -- is a big part of it.

For me, crossover ideas can come out of practically anything. I do not think your story about the club is weird at all.