I have seen a few stripped cranks in my time... the causes were (1)pedal not properly tightened; (2) cross-threaded or forced left side into right cranks; (3) excessive bearing friction.

(1) is always my first guess. But if the OP rode 100 miles then this is unlikely. Generally, an improperly installed pedal will pull out on the first few miles. It's not impossible, but just not how it has happened in my experience.
(2) is usually obvious, except where the threads are ripped out clean and no 'evidence' remains, but if someone is determined and careful it is possible to get a pedal in to a crank of the wrong thread type and ride it like that for a while. But the rider would notice or feel the pedal being crooked as it is usually pretty bad.
(3) does not happen too often, in my experience. If a pedal is properly tightened from the beginning, the amount of torque required to unscrew would easily be noticed by the rider as the pedal flips over under his foot.