I had the same yesterday. I sold a BitUSD and it has been bought instantaneously. This time, I observed the expired short and they are the ones buying my sell order.
I did some checking with some BitUSD, the "wall" is probably composed of expired and margin call shorts.
If you want to short and don't be bought directly so you have time to buy your own short order and roll over, you have to place it 10 % above the wall or above the highest margin call if there are.
In fact you can see more or less where is the 10% limit looking at the selling orders column. The cheaper one is above the limit (obviously, if not it would have been bought), so if you short slightly cheaper than it, you shouldn't have any problem.
You can also calculate the 10% and short above.
I use to sell 1 BitUSD as a test before roll over my shorts. If the BitUSD remain there unsold, you're good to go ! As long as the wall price/ price feed don't change, you can roll over the other shorts you have without having to use the 1 BitUSD test.
I understand it better when looking the order book on bitshgares blocks