Well heals generate threat, and tank stance has a massive multiplier. Shake it Off is a 400 party potency heal, and Equilibrium a 1200 self potency heal(with a HoT after) so that'll generate large amounts. Paladin can also do somewhat similar stuff by using Divine Veil and popping it during Requiescat window, since Divine Veil also heals for 400 now when it's applied to other people, but can't do as much self healing otherwise. And I guess if the WAR uses Bloodwhetting they also get additional healing out of that, you get like 400potency self heals per GCD x4, and you can force some of them to crit(infuriate attacks and primal rend will always crit the healing).
So maybe that'd explain it, threat is kind of a non issue anyway but warriors can likely generate more threat than any other tank currently due to all their healing.