Some fandoms just mesh well
. The timelines and continuities can be made to fit without much difficulty, they might have similar premises or ideologies, and they have characters that you just know would love (or hate) to meet each other. The chances of having them cross over increase if they're made by the same company, written by the same author, and/or have similar character designs.
Some fandoms are so accessible that they are crossed with just about everything else.
Related to Fandom Specific Plot
. A common form is the Doppelgänger Crossover