Wither Skeletons acts strange. As I already reported (MC-5565), them tend to walk in the edge of the structures of the Nether Fortress, what makes them fall when attacked.

But they also falls sometimes when they are in a place much higher than me and sees me: they jump and die. And they also appears to fall when they spawn in the edge of that incomplete parts that have irregular disposure of blocks.

In the images I attacched you can see that I was walking in other area and saw a fallen skeleton near to the incomplete structure. Or the skeleton spawned on the glowstone ou jumped from the Fortress without my interference and somehow stayed in the glowstone.