Like perhaps the weekly takings of any number of small businesses? It woukd be odd, but i dont see it as strange for people to wind up on planes with that sort of cash. Maybe they are just in a hurry and didnt get to the bank after work and plan to make the weekly deposit after the flight.

And dont forget that, at least for some people, 100k is chunk change. Maybe they are heading to a big poker tournament with other 0.1%ers.