Diagnosis
This is a design characteristic of two-leg (2-stage) desks with a single crossbar. The higher you go, the more leverage any movement has on the legs.
How to Fix It
Ensure all bolts in the leg assemblies, feet, and crossbar are tightened to specification with the included hex key. Most importantly, the desk must be on a level floor. Adjust the leveling feet on the bottom of each leg to eliminate any rock. A monitor arm mounted directly to the desk will reduce monitor shake.
If That Didn't Work
Consider adding a third-party stability brace or crossbar designed for your desk width. Also, keep heavier items (PC tower, printers) on a separate cart, not on the desk surface.
Heads Up
Do not overtighten the bolts connecting the tabletop to the frame. You can strip the particleboard. Snug is enough.