Hulkster,
This is according to one of the tamizh magazines...Apparently, at the time of pudhu pudhu arthangaL function, someone said it's purely because of Raja that the movie succeeded and the movie had nothing else other than good music. As an answer to that, KB chose not to work with Raja anymore.

KB's assistant, Vasanth, when he made poovellam kEttuppAr had this story incorporated in the movie...The script was interesting in that,
1. The split that happens between the MDs is based on MSV-TKR (drinking habits etc).
2. Vijaykumar, though he is supposed to be representing MSV, was made to look like Raja.
3. His fight with Karan (who portrays KB - the KB trademark vibhuthi streak sported by Karan) is Raja-KB fight