us-battle.net:
"the Sweeping Strikes attack in itself can proc Strikes of Opportunity, and in turn, that SoO causes its OWN Sweeping Strikes. This causes a chain of SoO and Sweeping Strikes until SoO stops proccing from a Sweeping Strikes. So hence, the more Mastery you have, the more Sweeping Strikes you'll net, as well."
icy-veins.com:
"The way things are looking this tier with most people going Fury single target and arms for AoE; Mastery pulls ahead because the extra swings from your mastery proc can proc off the damage done with sweeping strikes up and vice versa."
mmo-champion.com:
"Strikes of Opportunity can proc off of almost any ability, to include most special attacks and trinkets. Because of this there is an interesting mechanic between Strikes of Opportunity and Sweeping Strikes. Strikes of Opportunity will proc a Sweeping Strike cleave, which in turn can proc a Strike of Opportunity, continuing to chain from there. This chain will continue until one or the other fails to proc."