Learn Russian: Почему-то он не ответил.

The word почему asks the question 'why'. Adding the suffix -то turns it into an indefinite adverb. Here, it changes 'why' into 'for some reason'. It shows that a reason exists, but the speaker does not know or specify what it is.

Is the hyphen in почему-то strictly necessary?

Yes! The suffix -то must always be attached with a hyphen. Writing it as a separate word or joining it directly as one continuous word is a spelling mistake in Russian.

Why use the perfective verb ответил instead of the imperfective отвечал here?

We use the perfective ответил because we are talking about a single, specific event that failed to happen (he did not provide an answer). If you used the imperfective отвечал, it would focus on the process, meaning 'he was not answering' over a continuous period of time.

Can I change the word order and put почему-то at the end of the sentence?

Yes, Russian word order is flexible, so Он не ответил почему-то is perfectly correct. However, starting the sentence with Почему-то is very natural because it immediately highlights the speaker's confusion or surprise before stating the fact of what happened.