What makes tyres go out of shape?

I run a 57 reg Volvo S40 2.4i. At 39000 miles it developed a droning/booming noise related to road speed and I suspected a worn wheel bearing. My local garage diagnosed out-of-shape rear tyres and replacement cured the noise. 13000 miles on, the noise is back. What could be making my rear tyres go out of shape?

Asked on 28 October 2016 by Andrew Beal

Answered by Honest John
Pressures too low and being left parked for extended periods with something heavy in the boot.
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