I've had this happen a few times where a movement would be in the wrong axis. I would want a limb or in this case the tail to move downward in a straight direction (in one axis) and would key it so but for some reason it would move in another axis, looking like it is swivelling into position. This movement is unnatural in some cases and immediately looks weird.
Here is a clip of the problem:
I've tried deleting and re-keying which sometimes works but not always. I've sent this clip and the scene file to my tutor in hopes that she could shed some light on why this happens. Hopefully I'll be able to understand the problem and be able to fix it in the future.
The rig was made by Em Johnson, a class mate. I tried to make my own which didn't work out well and the previous rig that I found online was complicated for what it was and couldn't do certain things that I wanted it to be able to do, such as scale without effecting the tail.