As far as I know, this is an expression from Colombia, which is becoming popular in other regions through telenovelas and social networks. Even some people in Mexico are starting to use it regularly.
As I've seen it used, it asks for an explanation or shows disbelief/disapproval.
- ¿Ya te vas? Pero ¿cómo así? Aún es muy temprano. = ¿Por qué te vas tan temprano?
You're leaving already? But why? It's too early still.
- ¿Cómo así que no fuiste a la fiesta? = ¿Cómo que no fuiste a la fiesta?
What do you mean you didn't go to the party?
- ¿Cómo así que cancelaron el concierto?
How come did they cancel the concert?
By the way, "cómo así" is never put together to mean "like this". As Tomísimo said, it's just "así".
