Usage of "how come" while speaking

How come is a perfectly valid English construction meaning why.

The Online Etymology Dictionary records its use from 1848. It may be a little less formal, but it isn't invalid in any way.

How come you come to the office on Sundays? is grammatically correct.

"How come is this possible?" mixes up the word order, though - "How come this is possible?" would be better.

It is a normal part of spoken English, and even of some, mostly informal, writing. The OED’s earliest citation, in the sense, To come about, happen, turn out is from 1548:

How commeth this, that there are so many Newe Testamentes abrode?

I'd suggest it's a contraction of "how does it come to be."