We often trip over ourselves trying to minimize code behind and abstract behaviors in the UI from the models, etc. This is important for clean separation, but sometimes behaviors may add too much abstraction. The real fact is many applications require some sort of transition or animation based on events, and while we can try to put as many of those as possible into the
VisualStateManager, there may be instances such as dynamically created animations or special triggers that end up involving the view model somehow.
from Jeremy Likness more here