Laughter is provoked by a discontinuation of what your brain expects.
If you saw a dog walking a man on all fours, you would most likely laugh, because of the unexpectedness.
In this case, the unexpected terseness causes laughter. You don't expect what follows to just be "Eject". In a technical context like a manual, you expect what follows to be a detailed list of steps and instructions. When this expectation is shattered, you laugh.
Speaking of humour, I can't tell what I find funnier: the fact that someone didn't see how these checklist items could possibly be construed as humorous, or your clinically precise explanation of the concept of humour itself, complete with a mechanistic explanation and example. Together, they're a beautiful example of the tone of HN.
Everyone says they're funny because they're unexpected, but simple direction and plain language is exactly what I'd expect from an aviation checklist, so I have no idea.
It's not just they're unexpected. They're both expected and unexpected. You also expect a manual to have details. It's the sudden ending that is both fitting and not fitting that creates the humour shock.
Still, there is some humour in the laconic terseness and finality of that checklist, which is probably why it was brought up.