Hello, Vic.
There are no pull-up or pull-down resistors for the STEP pin on the carrier board. It is possible that (at some point) the Raspberry Pi GPIO pin is configured as a high impedance input, in which case the lead to the STEP pin (which is also a high impedance input) could act as an antenna and noise could cause the STEP pin to toggle. In this sort of situation, adding a 10k Ohm pull-down resistor to the STEP pin should force it to GND when the line is not being actively toggled. Generally, keeping leads as short as possible helps to reduce noise as well. Using a pull-down resistor on the SLEEP pin is also probably a good idea and should ensure that the driver sleeps when not being actively controlled.
The A4988 has low enough logic current draw that it might be possible to power VDD with one of the GPIO pins although, in general, that sort of thing is not a good practice and it does not sound like it is necessary to fix this problem.
-Nathan