The internet is a weird place. You can't really explain it. You just have to accept that at some point someone somewhere is coming to combine and mashup every possible combination of pop culture - and that it will probably be Disney-related.
And it doesn't have to be make any sense. All you can say is, "because internet."
Okay, so you might be thinking, "This isn't that big of a stretch. They probably made Elsa into the Winter Soldier, because ice powers." But you'd be wrong.
Elsa is Black Widow because she's a girl and Photoshop doesn't allow for gender swapping, apparently. Who gets to be the Winter Soldier then?
Hans. So we're not dealing with a logical mash-up.
Kristoff is Nick Fury? Sure, whatever.
Okay, this one actually kind of makes sense.