Looks like your surprised voice was enough to trigger her reactivity. Doesn't make it reasonable or professional of her, though. I hate having to tiptoe around people, but sounds a bit like she is one of them. Of course your boss doesn't have to explain her decisions, and she can just say, "Well, I don't want to go into that." She lost her cool. Not your fault. No boss should be blasting any of their workers!!

"Because I said so" isn't the best wording, but it's true - bosses make decisions, and we don't always know what crap they are juggling behind the scenes.