I don't see how it's even a question; your mother has the right to ground you. She gave birth to you, you live under her roof, and she provides for you. There is no reason that she is in the wrong for grounding you. If she wants to be authoritarian, it's her house, her rules, don't like it? Conform.
You do what she says, when she says it. The end. She is the one in charge of you, raising you and giving you the things she needs. When she calls you, its important to her. Your failure to comply is disrespectful, and your mom will not tolerate that behavior.
Your mother has a right to ask for help, but grounding you for asking why makes no sense. If you can't ask why then can you ask her anything? But overall, it may have had to do with your tone of voice when asking why, and if thats the case then it is fair.
If you have no right to ask questions, of anything (this has to be an absolutist position otherwise where do you draw the line logically?) then she can also ask the person who inquired here to jump off a cliff, starve him/herself and anything else among a pantheon of possibilities. Upon birth you are a human and have human rights. Despite what the radically conservative population of the US may tell you, people under 18 share those rights as well.
I've always subscribed to the notion that the goal of parenting is not to raise obedient children, but to raise competent, independent adults. "Why" is a very reasonable question to ask in response to being asked to do something, and is not deserving of any punishment. I don't agree with your mother's apparent parenting style at all. But ultimately you have to obey her until you're old enough to move out and support yourself.
It was an honest question of what she needed so they'd know why they are being asked to come down. The her grounding you is unfair is if she has say...Asked you to take out the trash, it's been like 5 hours and you still haven't done it and she's calling you down to do and you ask what she needs, that would make me pretty irritated. So yeah unless it was that situation being grounded for asking what she wanted is unfair