Parked.... outside..... downhill..... for months.....
Sell it or I am calling the Vehicular Humane Society. 
My VX does it every weekend. It's a Sunday driver so when I back it out of the garage and go, she has a slight "hop" to her. It lasts for a couple miles than goes away. I "ASSUME" it's due to sitting and the tires flatten on their resting side. I never thought much about it since it always corrects soon after. Perhaps heating them on the road and the centrifugal force helps alleviate the "out-of-round".
*shrug*
Gary Noonan
'01 S/C VX / '18 Forester XT