It’s failure due to liquid or debris. I’ve seen probably one or two defective buttons on an iPhone or iPad in the last 6 years. All the rest of the cases were accidental damage.

Along with the SIM tray, removal of the buttons reduces the amount of possible ingress by 3. All that’s left are the two speaker ports, Lightning port, and the receiver speaker.

With the way Apple is going with water/dustproofing, I am fully expecting USB-C on their phones to be a pipe dream or a short-lived memory.