No one actually gets it- that's part of the mystery!
There have been many theories over the years about it- some say it's perspective and that all the trees are leaning a certain way but that seems a little too far fetched. Why would every tree in the area just happen to lean one way? The original theory was that the hill was magnetic and that's why cars rolled up it, and that's how it got it's name.
I've driven past it a few times, but I don't think I have ever been there.
Def