@superkevdogg driver 3 and 4 Step/Dir signals do not share a common buffer IC but 2 and 3 or 4 and 5 do.
https://github.com/Duet3D/Duet3-Mainboard-6XD/blob/master/v1.0/Duet3_MB_6XD_Schematic_v1.0.pdf
Those buffers provide some protection but maybe you fed VMOT into them?