The bug

When you rotate an armor stand either by using the /tp or the /entitydata command, its base plate shakes during the rotation.

How to reproduce

1. Place an armor stand
2. Use the following command

   /tp @e[type=armor_stand,limit=1] ~ ~ ~ ~90 ~
   

A possible reason

The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.

As I am not very familiar with how the rendering exactly works the following is only an assumption and observation of what happens when you change something. The changes mentioned might only fix the symptoms of the bug, but not the bug itself.

It looks like the method net.minecraft.client.model.ModelArmorStand.setRotationAngles(float, float, float, float, float, float, Entity) is not calculating the value for the field net.minecraft.client.model.ModelRenderer.rotateAngleY correctly. Using for example Math.PI as constant prevents this bug from happening.

Keep in mind that these changes are no fix suggestions