Obviously they can do it as they have done it. Did they give you a reason?

I'm wondering too if getting the flight with miles put you in the front of the bump line.

I find it strange they would do it a day early instead of letting you standby just in case.

Are you sure they bumped you, that the plane is full and they don't have room for you, or did they cancel the flights for tomorrow and have to move you to another day? Two totally different things.

If it's a bump, then they should be offering you some compensation, dollars, free trip, etc.