There was news back in December about the DOJ in the US doing a probe into AMD and Nvidia. Link
I guess it is possible that these suits preceded that investigation but I would more guess that some litigators smelled blood when the DOJ probe hit the news.
In a duopoly it isn't hard to mutually arrive at certain price points without breaking any laws. The big question with the DOJ and these civil suits is if they actually conspired to fix prices and if they left a trail proving it.
I think it is just a simple matter of each company pricing competitively with each other. Not unlike gas stations simply lowering and raising prices as the station down the road does. ATI and Nvidia just like gas stations were able to smile at the dealer down the road as they crept the prices up the past few years.