I think it was Deanna or one of the others in Alexandria who blew the horn. Think about it, they went out to do a dry run. They got surprised at the quarry and as Abraham yelled when the truck at the top of the quarry collapsed, "we'll do it live!"

No one back at Alexandria, unless they had radios too, had any clue that the group was doing the Walker Pride Parade right then and there.

In the previews before the season, you see Alexandria under attack by some sort of humans. So it could have either been an attack by the wolves, maybe something akin to a warning note left via dead body at the gate or something. And with Rick and all the "fighters" gone, they blew the horn as a way of saying to Rick and company "get the fuck home now!", unaware that they were sending a giant ass herd from hell also headed their way.