The question is about the literal meaning of the joke, he knows what a pole is within the context of calculus. He didn't understand within the context of the joke what lead to the crash.
It's a joke. There is no cause within the literal context. The humor comes from word play.
After the plane crashed, one survivor was stranded in the wilderness. Miles from civilization, he cried and screamed until he got hoarse. Then he mounted the horse and rode back to civilization. Back at home, he found himself locked out of his house, since he lost his house keys with his luggage on the plane. He sat on the porch and sang various lamentations, until he found the right key and unlocked the door.
The joke relies on the fact that keys unlock doors, this is the cause, just like the prior one relies on the assumption that people rushing to one side crashes the plane.
If there are 10 comments explaining what a key is in the context of music and I add another that says "I understand what the key refers to but how was the door opened", one can only assume I lack the knowledge that keys open doors.
Sometimes people understand the more complex behaviours but somehow miss the simpler explanations, it's happened to me before.
But the plane didn’t crash because of weight imbalance - if the Poles all went to the left half of the plane, it wouldn’t have crashed, because that would make the system stable!
