Many drives (read TBxxxx and similar), when Enable is asserted, switch off their output stage and motors are unlocked, i.e. you can spin them by hand - no holding torque.
It is quite common that motors act "weird" while PC is booting, unless charge pump is enabled in Mach3.
Many years ago I made an interface between Mach3 and RZNC breakout board and added charge pump circuitry to be sure that machine will not play silly while booting.