Reasonable is in the eye of the beholder :) To him, that day, it was not reasonable. He feels that running back and forth is pointless, it hurts his legs, and it's not fun.

I get it, we can't simply refuse anything and everything that's not fun, but like I said, it was the first (and only - so far) time this year he had refused to do what he was told at school (to my knowledge anyway).

Perhaps his legs were sore and tired from basketball, perhaps he wasn't feeling well, perhaps he had just played soccer outside at recess and was not up for even more running, especially when he felt it was pointless.

I don't remember the exact reason, and I fully acknowledge and accept the reasonable points you are making. However when something like this happens (seemingly) out of the blue, there is usually a reason for it, beyond "they just didn't want to".