A quick trip to Wikipedia suggests that iPod Classics had a spinning disk for storage. The disk in hard drives "floats" on a cushion of air. When the outside air pressure gets lower, the internal pressure is relatively higher and the gap increases. A big enough pressure difference and the disc can jam and stop spinning altogether. I do not know if that is what affected your iPods, but I do know that hard drive failures at the summit of Mauna Kea (~4000m) are common and it seems to mostly be down to luck which ones work and which don't (once you have a working one, it tends to stay working).