Maybe its cause they are making a reference how the back of the car is round like a butt?

*shrugs*

Trying again as my comment hasn't shown after three hours...

I remember, well back in the day (as should Mark and Scott) when Warner Brothers used to do their "car of the future" cartoons. One of those had a pink car just for the ladies. The front kind of looked like cleavage and the back looked like her ass (similar to the pic).

Good times, that. Good times.