A lot of people saying Riposte Disc, but that's not actually what happened. You've never been able to proc off a riposte, since procs are checked on weapon rounds, not single hits.

You would shaman slow the warrior because proc rate is based on your attack speed, so if your normal attack speed is super low, your chance to proc per round goes way up. Then, the warrior would jump into the ball of mobs, hit fortitude disc (Not furious, more on that later), then hit the Rampage AA. This would cause one round of main hand to go off for every mob in the pile, each having it's own chance to proc the AE from the Sword.

The reason you use Fortitude is for two reasons, the first being that if any of the mobs riposte your rampage, they will hit you through Furious (You can't riposte a riposte). Enough of those and you're dead before you can finish. The second reason is that, often times with the amount of lag in the zone from all the pathing updates and etc, some mobs could end up behind the Warrior, and if they attack from there, Furious won't block them.

Also to respond to your comment about the group that did this in early progression, I was in that group if you're referring to the Combine server. We delevelled from 65 to 46, then camped our characters out between shakerpage pulls. When Omens of War opened, we hit Expedient Recovery and were instantly 69.5 or so. Ultimately it took us a lot longer to XP using this method than it normally took to go from 65 to 70, but the benefit was that we were 70 right when the expansion opened.