When Limitanei uses Shield Wall, they will lift their shield and keep it infront of them at all times. However, when you learn the technology Teardrop Shields it breaks the animation loop when they stop moving. This causes the Limitanei to put away their shields for a split second before lifting it again when moving about.
Here is an example of that using a Castle Age Limitanei with the Teardrop Shields technology.
And here is a Castle Age Limitanei without Teardrop Shields showing how it is suppoused to look.
I have tested this and found it to be the Teardrop Shields technology, since it will not happen in any other way. Dark Age, Feudal Age, Castle Age and Imperial Age upgrades do not cause it, only the Teardrop Shields technology will.
It makes the unit look very jittery so I hope this can get fixed.