Only the class that declares the event can fire it. It's not possible to fire events from outside that class, unless a specific method was exposed on it.
The event fires if a property on the layer changes. Since the layer owns its properties, it will fire it when it needs to, so there shouldn't be a reason for having to fire it from outside of it (but please do share why you need this).
Note that this event is only provided to support data binding scenarios. Generally changing DependencyProperties doesn't fire this event because it has other means for trigger rebinding.
