Originally Posted by Red__Zed

The short explanation is that there is some overlap between intake & exhaust valves opening. There is vacuum in the cylinder, and backwards airflow (backflow) is pretty common, generally sucking some small amount of exhaust gas back into the cylinder. In cases where a cat (again, generally a pre-cat because of their proximity to the exhaust ports) has broken down, you have
A) loose pieces of honeycomb floating around
B) significant increases in backflow

and it becomes technically possible for cat material to flow back into the engine.