The passenger/vehicle entities have been switched for both the EntityMountEvent and EntityDismountEvent.

For the EntityMountEvent:

• event.getEntity() returns the 'vehicle'
• event.getMount() returns the entity that is mounting the 'vehicle'

For the EntityDisountEvent:

• event.getEntity() returns the 'vehicle'
• event.getDismounted() returns the entity that was riding on the 'vehicle'

In 1.8, these were reversed.