Steering wheel blistering

From what I've picked up from reading on here and various other forums, Renault steering wheels are appalling for blistering melting and bubbling and that its fairly common, But what causes it, is it the car being left in direct sunlight?

My cars a 59 plate with 33K, bought it a year ago with 17K and the steering wheel was perfect, so I just cannot work out how for the 1st 2 and a half years of its life the wheel remained perfect but its gone from that to being all bubbled and rippled in under a year.
 
I've had three clios with a combined age of 7 years iirc and not one has had this problem