Old thread I know, but I think I know the answer and since there are old machines out there, I will share what I found out. I had exactly the same experience on a 2008 machine.
It comes from trying to make it go faster than it was designed to run.

This was from emails from CNC-Step.

"The machine is from 2008 and maybe you have spindles with 3mm distance per revolution, and the max speed is 15mm/sec for this kind of spindles. "

"the newest version of the High-Z machines comes with Trapez spindles with 6mm ( distance per revolution ) and the max. speed is 50mm/sec."

"The newest generation of the High-Z CNC router comes with better spindles, the stepper motors and the axis controller is stronger than on the machine which you was trying."