The current theory as endorsed by Blizzard is that the Worgen originally came about due to a Druidic order who specifically focused on wolves. Just as we have Druids of the Talon who shift into birds and Druids of the Claw who shift into bears, there was once (we presume they no longer really exist) a sect of Druids who shifted into wolves or worgs.
Now, suppose this Order was somehow different to the others, or was changed in some way to give them a heightened awareness of the arcane. Maybe they were closer to the Elven Mages of that time, or were affected by the Legion’s magic somehow when they were testing the waters before the Sundering, so to speak. Whatever the cause, they then could have had a vision of the very distant future.
In this future, the Burning Legion has invaded Azeroth. The mortal races are either being wiped out or already extinct, and the world is generally on the edge of destruction. Perhaps the defeat of the Titan watcher inside Ulduar, Algalon, has meant that the Titans haven’t been alerted to this.
The Druids realise this can’t be allowed to happen, and set about finding a way to stop the invasion. They make some sort of pact with the Bronze Dragonflight, perhaps with the aid of Ysera, the Green Dragonflight Aspect who is close to the Druids, and get transported into the future.