It could be the Captain Chaos rule if it actually had a name. Kenny Koretsky is the only driver to have qualified at an NHRA national event in a Top Fuel dragster and a Pro Stocker. Not long after this feat, NHRA created a rule mandating that a driver cannot compete in two professional categories at the same event. 

However, a pro driver can race a sportsman or a specialty series. Even a sportsman driver can compete in two classes at the same event. It's no secret that Matt Smith plans to help Mike Salinas and his brother Carl get licensed over the winter. 

One has to wonder... what if Salinas wants to race the Pro Stock Motorcycle at an event, and it's in the regular season? A drag racer can miss an event in a car and if they do well enough can make the Countdown. Would NHRA be so headstrong in their traditions that they'd be willing to lose a Top Fuel dragster for a bike?