Try the project with a thinner board and see if you get past that point... don't let it carve.

Hole in the Brass Roller
Dusty Z Encoder
Bad FFC or FSC Cable... (same thing)
Bit installed TOO long or too short?????
Carve will be TOO Deep, machine "Tilt" for the young guys.... It's a old Pin Ball Game thing.... When you shake the machine.

AL