In previous editions, "backyard" was the form for adjectives — as in "backyard barbecue." "Back yard" was the place — "We ate barbecue in the back yard."
The old rule may have required thought, but it made sense. It preserved the notion of a "yard" as a separate place. You can have one both in front or in back (or to the side, I suppose) of your home. We don't have "frontyards," we have "front yards."
The single-word adjectival form simply acknowledged that the words "back" and "yard" had grown close together in common usage.
The AP gave into people who didn't want to learn a logical, simple rule. It chose convenience over sense.
I know the change has been out for a while, and folks have probably discussed it. But I just happened to be thinking about it, and thought I'd vent. Thanks.