In this scenario, one or perhaps both major axis powers actually carry out a plan to destroy the canal locks in Panama during WWII..
It can be any which way(be reasonable though).. Also how much of a impact does this have on the U.S war effort, to what degree?
Maybe Japan is really really bold and gets those canal locks as a "bonus" during Dec 7 1941. Midget Sub attacks?
Remember all it takes is one lock out of commission to really screw things up good..