My knowledge on the A4988 is limited to a couple of minutes googling, but you seem to have a standard Step/Direction signal from the controller (Uno). It sounds as though the Y and Z "Step" pins are acting together. In the first instance I'd check continuity between the Step input on both A4988s (Y/Z) - there shouldn't be any. I'm equally not familiar with a GRBL setup on the Uno, though I have a working knowledge generally of Arduino and similar SBC/microcontrollers. Somewhere you would have to bind the Uno discrete outputs for the X/Y/Z step/direction outputs to the output pins of the Uno, and ensure that these output pins drive the A4988s. Check all that.

It could possibly be the software running on the Uno - though if this is a public-domain GRBL sketch then I would not expect this error to have propagated unnoticed.