You got me thinking - now I think I know what might have happened!
After shooting, I left both cameras on the counter at room temperature for 14 or so hours BEFORE I rewound them. I know little bits/droplets of water did get on both cameras while shooting ( I'd quickly wipe them away).
Perhaps during those 14 hours there was moisture allready inside the camera, and it made it's way into film, spool, or cartridge, though I didn't notice any such thing after opening up the camera after rewinding. The filmback isn't exactly water tight either.