Garrosh gets on Theron's bad side when he throws the blood elves to the wolves in 5.1, not bothering to warn them about hostile presences and mogu ambushes. It gets worse when Lor'themar has to quite plainly spell out why Garrosh's new role models (the mogu) are not the sort of characters the Horde should be emulating, to which Garrosh essentially says, "Did the Scourge beat all of the vigor out of you? Shut up."
Anyway, when Garrosh orders Theron to toy around with some new artifact he's found, it goes badly wrong and releases a sha on Silvermoon. The players stop it, but Theron has enough of Garrosh's shit and considers looking at "old Alliances" if this continues.
Theron and Varian start talking about just that, but that's when Jaina begins her purge of the Sunreavers and Rommath/the players attack Dalaran to save them, ending that option too. Varian has a fit and says that Jaina's pushed the blood elves back into the arms of the Horde by attacking their people, when he was about to negotiate Quel'Thalas back into the Alliance.
Lor'themar says pretty much the same back in Silvermoon, goes on a massive rant about Garrosh and the Alliance, and decides to take matters into his own hands.
It's basically seeding the blood elves as members of the Horde rebellion, in other words -- putting them firmly at odds with both factions until the better option comes along.