It took a while for me to wrap my head around that.
Isn't that kinda weird?
Every orc who is now in the Horde refused to follow Garrosh. They at some point chose to stand against him. But there are orcs now willingly following Sylvanas. Like the orc in that cinematic.
So there are orcs who went:
"Nope, that brown orc, he's dropping manabombs on our enemies, that is going too far...
But this dead lady, now that's my kind of lady, she's throwing around weird green stuff, I love it."
I think that's weird.