Originally Posted by ThreeJulietTango

The reason does make a difference in determining your priority on the standby list for the next flight. AA has begun giving higher priority to people who arrived at the airport on time but missed their flight due to customs/security lines, versus people who simply arrived late altogether. AA will also let you standby for a later flight on routes not normally eligible for standby if you missed it because of CBP/TSA.

As others have cautioned, blaming TSA may not be effective if it's a quiet time, but that will only affect the OP's placement on the priority list, not whether or not they will be allowed to standby (unless the flight is not domestic or to/from Canada or the Caribbean).