It's because your camera has a X-Sync speed of 1/250 or 1/200. When the
metering in AV mode wants to make the
shutter speed exceed that to get a correctly exposed photo, the camera will prevent the
shutter speed from going faster. This will cause you image to be very bright (usually completely blown when outdoors).
What you can try is use High Speed Sync. Press the button with the "Lightning symbol with H" on your 430EXII. This has its own limitations such as reduced power and range but will overcome your problem of blown out shots.