Evo live wallpaper and a magnet?

I have an HTC EVO 4G and I currently have a live wallpaper that's like little particles swirling around. I just got a belt holster for it for work and it has a magnetic closure. If I set my phone on top of it with the phone unlocked and on the home screen, the wallpaper kind of... gets stuck? And I take it off and it goes back to normal. I thought I was imagining things but I tested it several times with the same result. Any idea what could be causing this? Thanks :D