This is a normal issue on G series Kirbys that see heavy use, or more extreme use of the drive pedal. Every time you press the pedal, it wears away at the cam grip minutely by nanometers. Over time, that adds up more and more wear on the metal pin until the camshaft doesn't grip anymore and it just falls out. It's the danger of using press-fit parts and not securing them more strongly as a backup hold-down.
I have two G series Kirbys that have this issue, plus my own personal Ultimate G owned since new in 2002. On that one, the front wheels came off the same way. No amount of pressing them back on again got them to stay on, they just worked themselves loose again. I had to replace the entire axle and wheel assembly to get fresh metal. I bought a second axle just to satisfy my curiosity and tried to fit my old wheel set on it. Well, they wobbled around and fit pretty loose, and when I rolled it across the carpet as a mock demonstration, the wheels just slid further and further off with each forward and backward roll until they fell off.
