The pairing could originate from the episode Bebe's Boobs Destroy Society, where Cartman and the rest of the 4th grade boys became attracted to Bebe because of her breasts. Cartman invited Bebe round to his house - much to the chagrin of the other boys - where they played out The Silence of the Lambs with Cartman's toys.
In Stupid Spoiled Whore Video Playset, Cartman also tried unsuccessfully to get himself invited to Bebe's stupid spoiled whore party.
Bebe placed Cartman higher then Kyle on The List in the episode of the same name.
Bebe egged Wendy on in the classroom to fight Cartman in Breast Cancer Show Ever, it could be possible that Bebe just wanted to see Cartman fight.
More recently in The Ungroundable Bebe killed Cartman's persona in Call of Duty where she turned around in her chair and gave the finger to his back. Cartman then killed Bebe with a cry of "You got pwned you Jap bitch!"