I'm not sure this is the reason but one other "crew scheduling" issue could be related to FAA Notice 8900.304 ("Special Pilot-In-Command Qualification Airport List") -- Burbank is on the list due to "Mountainous Terrain".

It is possible that the crew realized that it was a special qualification airport and the PIC in command didn't meet the qualification for one of various reasons, including, e.g. that the didn't meet the recency requirements for the last time they flew into that airport.

If we want other "weirdest return to gate" -- I had a 737-800 IAH-GCM a couple years ago that was holding short of the runway for departure and announced we were returning to gate because "none of the radios are working" -- how we got to the runway threshold without working radios is one of those questions I've always had in the back of my mind...