How can Warner Music Group claim copyright infringement on a Mozart Symphony? Isn't that public domain?

My Friend and I have a youtube page. We made a video that contained a Mozart Symphony (can't remember the name at the moment) The video was flagged by WMG for copyright infringement, and youtube wont let us challenge it, so we don't know what we infringed on. We chose Mozart to avoid copyright infringement, I'm wondering if there is something I'm missing here.