Usually Warped Rotors are caused by too much heat IE: Hard braking all the time, riding the brakes.. etc.. But 35,000Km on brakes is horrible IMHO.. I've owned my car for 5 years and put on about 70,000KM since I bought it and I only changed the back brakes once, cause the pads were beyond finished. The fronts are still the ones the previous owner put on... And the rule for brakes is 2:1, every 2nd time you do your fronts you do your rears.
I believe that some Rotors are directional but not 100% sure.
If it is heat, you might want to check your wheel baring to make sure that it turns good, if not it can cause everything to heat up.
1994 318is-Sold July 13th 2010.. Sad Sad Day