Learn English – I had it coming

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's invariably used in fairly lame "witty trading name/slogan" contexts, which as others have pointed out, all derive from the original "Toys R Us". The noun that precedes it is always something "we" (not "us") specialise in supplying.

I can easily imagine someone in a group of enthusiastic drinking buddies saying, for example "Alkies are us!", but after Googling "are us" -shopping and leafing through several pages of results, I have to say it's not exactly a commonplace idiomatic usage outside commercial contexts (that's to say, I didn't come across a single instance of anything 'non-commerical').

Grammatically speaking it's, well, not. You can say, for example, "We are human" in standard English, and "Human are we" in Yoda-speak, but "Human[s] are us" is never going to cut it.