So what was actually up with Maddies visions?

Why was she seeing the things she was seeing? She was sitting in xaviers truck outside of the hospital when Simon's body fell onto the truck. I was so scared during the last episode when her and Simon were falling, that Simon was just going to fall and land on Xaviers truck like her earlier vision! Im so glad that didn't happen, but I wonder why she had visions like that?