Any capacitive coupling of the AC power, differences in ground currents, etc to can cause a small AC offset between that signal or ground and the point in the breaker box where all of the grounds are ties together. When everything is one phase of your wiring these small offsets all go the same direction at the same times so the differences between them are smaller. When some equipment is on a different phase the differences in the small offsets are larger. Those differences can flow over the grounds in the interconnects between your boxes and any unit that doesn’t have perfect common mode rejection on it’s inputs will have a small amount of hum.
Put in a slightly different way, nothing is perfect but having the AC power related imperfections of each of your components track each other rather than going in different directions lessens any effects of the differences between them.
You can use a multimeter to measure the voltage between the hot sides of the two circuits to see if they are in phase.