DC motor interfering with another
I see that you reckon that you've found the cause but it is possible for one motor to act as a generator and power the other.

Figure 1. When turned by hand the motor will act as a generator and current will flow in through the intrinsic MOSFET diodes (not shown on the schematic symbols) as shown by the red arrows. The reverse polarity MOSFET at (2) is good at preventing reverse polarity but not reverse current. Since its gate is pulled low at (1) it may turn on and pass current out onto your 8 V bus.
This in turn powers up the other board which may run depending on what happens its microcontroller when it gets some unexpected and badly regulated voltage.