"My mom was a teacher at my high school and other students were constantly asking me for test answers even though I had no access to them. One guy – who always insisted I had access to the test answers – told me my mom was giving out a multiple choice test and asked if I could get him the answers. I told him I could, and a few days later gave him the ‘answer key.’ Truth was I just typed a bunch of A, B, C, and Ds on to a paper and did my best to make it look legit.
He saw me in the hall after the test and said, ‘Hey thanks!’ and winked. I replied, ‘Yeah, I made all those answers up. You probably failed.’ His face dropped quick. I told my mom later. She laughed. He did fail. Don’t cheat, kids."
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.