Good option: spoilers:end of spoilers
Emma did it, because, well.. she's Emma and she enjoys messing with Firestar. Or maybe it's her way to say "sorry for your horse"
Bad option: spoilers:end of spoilers
Emma did it because Firestar is actually a traitor and she's setting a trap for her and whoever she's spying for. I hope that's not the case, because that would be Duggan giving bigotted fans what they want. But, eh, he's the guy who came with X-Men Green.