¿Cómo están todos?
 

I heard this on a tv program. Why is the "todos" there, since it's obviously plural due to the verb form?

The subject 'todos' isn't the same subject as 'ustedes', 'Juan y María' or any other plural subject you can think of. When the subject isn't provided, it is only understood by context. In other words, if the subject hasn't been established, or if it isn't understood, it needs to be stated.

everybody

What's more:

todos = everybody

todo = all

:)

@Caballero: I assume that the speaker is asking his/her audience how they all are. In that case, "todos" is an ellipsis of "todos ustedes".
¿Cómo están todos (ustedes)? -> How are you all?

If the person is talking about someone that's not there, like in the case of asking how someone's family is doing, "todos" is referring to "todos ellos".
¿Cómo están todos (ellos --tus hijos, los amigos, nuestros tíos...)? -> How are they all?

¿Cómo están todos?
¿Como están ustedes?
¿Cómo están todos ustedes?

or just, ¿cómo están?

All are correct and said and mean just about the same.

"Todos" works there as a pronoun. It's not the short of something nor an alternative to pronouns as "ustedes/vosotros" the same way "everybody" is not the same as "you (plural)". As a pronoun it is used to speak to large publics or even to masses. It is the pronoun chosen by a person who addresses a lot of people but can't look in one direction and make visual contact with all of them (at the same time).

"¿¡Cómo están ustedes!?" (those who are making visual contact with me)
"¿¡Cómo están todos!?" (everybody, no matter they are making visual contact with me)

About television or radio, both are the same if the person who greats is addressing the audience, and in TV, looking to the camera. If not, "todos" is the choice.

Everybody have beat me in answer this question, but I want to add something else in this case, the word necessarily means Everyone, when you need to express or expose something with all the people present in that moment is necessarily use the word Todos unlike that of other word used for this kind to phrases but I don't know other word for that expression.

Sincerely yours.