What is the eye wall and what causes it formation?
The wall of cumulonimbus cloud surrounding the eye of a hurricane.
What you actually want to know is what causes the formation of the eye of a hurricane, as the wall is simply where the storm begins again.
Wind is caused by a pressure gradient, or a change in pressure over a distance. The stronger the gradient the stronger the wind, so as you can imagine the pressure gradient of a hurricane is extremely strong. That being said there still has to be a low pressure center, the point with the lowest pressure. As the low pressure center is not a singular point but an area that can be several miles wide of low but uniform pressure.
Since the eye has relatively uniform pressure there is no gradient, and therefore no wind. There is a bit more to it but basically this is why there is an eye of calm weather in the center of a hurricane.