"In a dorm prank war my freshman year of college, I screwed off the shower head, put a couple chicken bouillon cubes inside, and put the head back on. Then I sprinkled blue Jello mix powder onto the towels. Next thing I knew, some guy was yelling about chicken soup and stained blue like a Smurf from drying off with his towel."
If it is really old and rusted pipes it can happen randomly, but typicaly after peaking pressure that will rattle the dirt in the pipes loose. Reconstruction where the pipes are knocked can also do this.
I don't think mom actually immediately thought of the exact cause, people...
She just thought something was obviously wrong which was the whole point. Best to move on, future plumbers.