The reason I asked what method you were using for casing is because you mentioned that you cased by pulling your leather through a pan of water and based on that comment you are just wetting the leather but not casing it. Many people mistakenly use the term "casing" for wetting but I didn't want to assume that was the case here. The casing process can be achieved with some variations, such as using additives or using casing boxes ILO bags, etc., but the bottom line is that leather is either cased or it isn't. The process doesn't change. It doesn't matter if I'm using W&C, HO, Thoroughbred or Tandy leather, the casing process is the same. So if my assumption is correct and you are simply wetting your leather, you will benefit greatly by casing. However if you are truly casing your leather then there are a couple of things you might try.
The first is to re-sharpen your swivel knife blade. You would be amazed at how many people cut with dull blades and don't know it and that's understandable because if a person has never had a sharp blade how would they know what sharp really is? As Brent mentioned, the chattering you feel is corrosion forming on the blade due to the residual chemicals in the leather. Sometimes you get a side which is just difficult to cut and requires a lot of stropping (which does dull your blade, BTW).
So the next thing to try is adding a small amount of Joy dish washing detergent and a small amount of liquid glycerine to your casing solution. The soap is a surfactant and helps the leather absorb the water more completely and the glycerine seems to help the leather to cut a little smoother. I think it's acting like a lubricant, but I can't prove that. Additionally when I have a side which is difficult to cut I will apply a light lather of Ivory bar soap to the surface of the leather, again, to act as a lubricant while cutting.
With all that said, I rarely need to apply anything to the surface of the leather when using cased leather. So anyway, hopefully some of this will help, however, if I have made any assumptions in error I apologize and please ignore me!
You said <<adding a small amount of Joy dish washing detergent and a small amount of liquid glycerine to your casing solution>>
Can you tell me how much is the amount of dish washing detergent and glycerine you add to the water of the casing solution?
Thank you in advance.