I think it has something to do with the frequency your suspension and related components are exhibiting.
I had this fatiguing issue when I was lowering my car. I ran out of time and had only lowered the front and was driving around like this for a few days. I noticed that the steering exhibited this same high frequency vibration. At the same time I felt like I had to make a lot of minute steering adjustments on the highway. It was very tiring to drive. I'm glad that installing the rear suspension alleviated this issue.