Holden Astra 2002: What is causing uneven tyre wear?

I DRIVE a 2002 Astra with Dunlop SP 200E 195/60 R15 tyres. I have driven 37,000km. The car is generally quiet, but in the past 2000km I have noticed an increasingly loud humming-groaning noise from the rear. This is especially loud and frustrating when driving slowly or when braking. I took my car to a Holden dealer and they said it was the rear tyres, which had worn unevenly in a "corrugated" way. The dealer changed the tyres and the noise disappeared. I am a sensible driver and drive only on bitumen roads. How does this happen? How do you avoid getting these corrugations? Is this a common fault for the car or this brand of tyre? Is there a quality, quieter tyre (up to $140) you can suggest?