Why is "head for" in the passive voice in "Mitt Romney was headed for a decisive victory in Arizona”?

Today’s Washington Post article titled, “Romney wins big in Arizona, AP says” begins with the following sentence:

Former Massachusetts governor Mitt Romney was headed for a decisive victory in Arizona Tuesday night, where he had the support of the state’s governor, Jan Brewer, and a sizable contingent of Mormon voters.

I am interested in why it isn’t “Mitt Romney headed for a decisive victory,” in the active voice as “Romney wins big in Arizona.”

If I put it “Romney headed for a victory,” am I making a serious grammatical error?

• the phrase

X was headed for...

is not in the passive voice. It does superficially look like a passive: 'X was sent' means some agent 'sent' X somewhere. But here 'headed' is acting like an adjective.

• the sentence:

Romney headed for a victory.

is not grammatically incorrect at all, just not particularly natural sounding and that is why it was not used. 'head' here, as a verb, is allowed but as in 'I go ' vs 'I am going', the latter progressive is much, much more common. 'X headed for Y' is very natural sounding when Y is a location; here it doesn't sound right because 'Y' is not literally a location (though it works metaphorically just fine in 'X was headed for a victory').

The usage you mention is typical for election-evening reporting of ephemeral results. That is, when the headline appeared online a few hours ago, or perhaps when the printed paper hit the streets, the Arizona polls had not yet closed and the official result had yet to be declared. If the result is no longer in doubt when the article is read, "was headed" will be more accurate than "is headed". For example, articles available now appear not to include the "was headed" wording, instead leading with "Mitt Romney won the Arizona primary Tuesday".