Et in pecunia ego
For many of us, Holy Blood, Holy Grail was our introduction into the Grail arcana until Brown's novel came out. The deaths, the intrigue and the involvment of British Intelligence and (DeGaulle's old friend) the mysterious Pierre Plantard du St. Clair make the story of HBHG almost as interesting as the book itself. By comparison, The Da Vinci Code is rather silly and Hollywoodish, hardly something that anybody with two neurons to rub together would take any more seriously than a dumb funny.
One could only speculate that Bagient and Leigh might want a piece of the multi-million dollar payday the film is bound to generate. After all, Brown's novel has been out for three years now. The authors had all that time to file a suit. And it's not like they could have just now discovered the book, since coverage and analysis of it have happened everywhere around the world.
The third Holy Blood, Holy Grail author, former BBC journalist Henry Lincoln, was not mentioned in the suit. Then again, he must have made a couple of bucks with his videos about The Da Vinci Code.
Maybe the whole thing's a stunt to drive up interest in the movie.