I can relate to your experience! I also have a Pom (7 yrs old) who is a little snot! we love her, don't get me wrong, but she does like to instigate trouble! Darmok just ignores her, so she leaves Dar alone, but she can get Sasha riled up soooo easily, and Sasha retalliates with a stiff-legged jumping growl that turns into a bark fest, then she and the Pom are snarling lips at each other! Luckily, Sasha does not try to fling Heidi (the Pom) across the room like WIllow did, but it's still dramatic! It's not an everyday occurrance, and never lasts more than a few seconds, but it drives me crazy when they do it. We always leave the dogs at home together, and if we are gone, Heidi goes in another room and goes to sleep. I really think it's just an attention-getting thing they do to see how we will react!
You say you talked to a trainer? For obedience classes, or what? Maybe that will help. Maybe the Pom snapped first while they were behind your back and Willow was just "defending" herself?