I've uploaded a sample blend file.
The action constraint evaluated in the scale transform is giving wrong results.
As you will see in the blend file, bone_3 has and action constraint that is triggered with bone_2 Y scale in local space.
If you rotate or scale Bone_2, you'll notice that the action is triggered randomly. Even if you rotate or scale Bone_1, which is the parent of Bone_2, action will be strangely triggered. Location transforms does not seem to trigger the constraint.