The more I re-watch the video, the more it looks to me (in slo-mo, at least) like both runners were actually altering their course, lowering their shoulder and trying to hit Crosby. She shouldn't have been as close to home plate as she was, but it is really starting to look to me like both of those girls were looking for trouble, and they found it.
In fact, on the second girl, it really looks like that if Megan didn't throw the elbow, she would have ended up on her arse instead of the runner.
If you look at the pic taken from CF, you can see how much she was leaning her body into it, too. It wasn't just her elbow. The catcher built the foundation to be a brick wall.