In order to achieve your expected result please attach to chart's MouseWheel event like shown below:
void chart_MouseWheel(object sender, MouseWheelEventArgs e)
var pos = e.GetPosition(this.chart);
ChartPanAndZoomBehavior behav = (ChartPanAndZoomBehavior)this.chart.Behaviors.FirstOrDefault(b => b is ChartPanAndZoomBehavior);
bool isZoom = behav != null;
e.Handled = true;
This will make sure that the chart will zoom only when the mouse is over it. Otherwise, scrolling outside of the chart area will scroll the ScrollViewer.
Attached is a sample project demonstrating the above approach.
Hope it helps.