Some of the culprits:
Cell phones - although mine doesn't do it, a companion's did once
Digital Cameras - the regular film kind don't cause issues
Anything magnetic - magnets on wallets, purses, keychains, magnets you bought for the fridge at home
Eelskin wallets
Keys - not really that they erase them but they scratch them and it has the same end result
The magnetic strips on the key cards are really quite a bit more sensitive than regular credit cards, I have learned the hard way. Of course, if you have the dead one then the desk at your resort can re-outfit you with a new one. And, if it dies mid-park and you need it for fastpasses then you can get a replacement ticket printed by guest services in the park.