Steppers motor start moving without any commads being given

We have built a CNC machine using open-loop stepper motors. The motor drivers are connected to a computer via a breakout board, which is linked using a DB25 cable. Most of the wiring is done using jumper wires, and we have a common ground for all electronic components. The Y-axis is driven by two stepper motors, one for each arm.To improve signal stability, we have added pull-down resistors to each Step- pin of the motor drivers. However, we have observed that the potential difference between the 5V pin and ground pin of the breakout board (powered by an 18V power supply) is only 1.6V. Additionally, when the Y-axis motor stops working, the potential difference between the 5V pin and the Y-axis direction pin drops to 0.06V.Thus, we suspect that the breakout board is damaged. The 5V pin on the breakout board should provide a stable 5V, but since it only outputs 1.6V, the board’s voltage regulator or power circuit may be faulty. The fact that the Y-axis direction pin drops to 0.06V when the motor stops further suggests a power issue or weak signal.Additionally, we suspect that electrical noise is affecting system stability, leading to random motor movements. Interestingly, noise issues only appear when the PC is connected to the breakout board. What could be the reason for the issue? Should we also connect breakout board with a usb cable to the PC for extra power. YOU can view our breakout board specifications in the file attached below. 

Try to measure potential difference bethween the pc ground and stepper drivers ground, if this card not have optical isolation on the step-dir signals, its cause problems.

AFAIK those breakouts do not have a 5V regulator, you must supply 5V.
So the behavior you see is pretty much as expected. That is, the 5V side is being
powered (weakly) via the PC parallel port data pins.

The regulator just supplies the inputs and the analog spindle interface
(regulates to 12V)

Thank you, everyone, for your responses. Really appreciate it!
We had already connected the negative direction and step pins of the TB6600 driver to ground using a pull-down resistor. After taking PCW's advice and supplying 5V to our breakout board via the USB cable, all our problems with erroneous movements and strange noises disappeared.

Points to note, though:

We had already made the ground common across the whole system.

Optocouplers are pre-installed on this breakout board.