Usage of "how come" while speaking

I had always heard people saying like (never seen written anywhere but just heard it):

How come you come to office on Sunday?
How come is this possible?

It doesn't seem right to me when people say this but is this valid in English or valid just for speaking?

Can anybody shed some light on this?

Solution 1:

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.

Solution 2:

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?

Solution 3:

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