Both your memorytile
divs are set to the same z-index but because tile2 is after tile1 it is treated as being above it. memory is within tile1 (which is now below tile2 and so you get the effect you are seeing.
Change the z-index of tile1 to be higher than tile2 and it will work. I updated your fiddle: http://jsfiddle.net/RkpAe/2/
There is also a nice explanation of this on http://webdesign.tutsplus.com/