"I pulled a prank on my dad where I left photos of Nicholas Cage EVERYWHERE. The kitchen cabinets, oven, fridge, toilet seat cover, in his book. I probably ended up printing about a hundred Nicholas Cage faces total. Just when my dad thought he found all of them, he was pulling out of the garage and almost had a heart attack when he saw Nicholas Cage in his car camera."
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.