Looks like the rust had a role to play, but the lesson is not to go mental with the power washer! I only use it at full power on the wheels, never on the paint or body - I learnt that lesson after I removed bits of the vinyl boot sticker on my last MK2.
You're only wetting/rinsing the car with the power washer, not cleaning it. If you want to get more dirt off before washing it, then invest in a snow foam lance.
