Is the structure of this sentence "Why may standing up for a long time cause hypotension?" correct?
It is grammatical but, in a medical scenario I would use "how" instead of "why", and "can" instead of "may". It's just that hardly anybody uses "may" in ordinary conversation (or in medical schools) in AmE.
Standing up for a long time can cause hypotension.
How can standing up for a long time cause hypotension?
Some people are prone to hypotension when they keep in the standing position for a long time. Several factors can/may cause it.
First, and most important,
Why may standing up for a long time cause hypotension?
is a rhetorical question. No native English speaker would ever ask it as a real information-seeking question, unless prompted by some prior statement to use that ungrammatical construction with epistemic may.
Second, if it's a rhetorical question, then the important thing is the answer, not the question.
And the answer, viz
Standing up for a long time may cause hypotension because
is a normal use of epistemic may, from which the rhetoric question was crafted, via the normal rule of Wh-Question Formation. Except that the craftsman did not realize that many constructions with various epistemic and deontic modal auxiliaries are banned in some environments.
Like epistemic may, which is ungrammatical in questions.
- This may be the place. ==> *May this be the place? ==> *Why may this be the place?
I'd normally say something like Can this be the place? or What makes you think that?, instead.
If the sentence is asking for information, I think using "might" or "could" instead of "may" would be more correct. If it is like a blog title or something, and you want it to be a grammatically correct sentence, it should be something like "Learn why standing may..." or "Find out why..." If you don't care about grammar, the trendy "BuzzFeed" way of doing it would be "Why standing may..."