Simple, maximizing profits and market control. Those of us that have been around awhile already know that price fixing is taking place on some level between AMD/ATI and Nvidia. We know this because of memos between the two companies discussing it a couple of years ago being leaked, and they taking some heat over it. Because it is just the two of them that control the market, more Nvidia than AMD, they don't have to be innovative, or rather, release their innovative products. A good example of this....
AMD releases the 7xxx series, while a powerful card it wasn't the powerhouse that people thought it was going to be. Nvidia seeing this decides to release the GK104 as the GTX 680, and last night, the GTX 690 announcement using the same mid tier GK104. Why was this done? Nvidia didn't have to release the GK107 or GK110, because their GK104 was just as good as what AMD put out.
**Personal speculation at this point: We will see GK107/GK110 as a 700 series card. This will allow Nvidia to maximize profits by being able to stretch their Kepler line further, as they didn't have to come out of the gate swinging. They can sandbag the GK107/110 for later release, giving them more time between now and Maxwell.
If we had a third player in the game then we would see more customer friendly pricing, or at least more innovation making it out the door to the customer for the same money. Like 3DFX with the Voodoo line, god I loved those guys! That isn't the point, however, the point is that without strong competition the market will become stagnant to a degree.
But to more directly answer the OP's question, it really isn't insider trading. It is simply trading of information, as both companies understand that if one came out with a card leagues upon leagues beyond the other, it would destroy the weaker of the two. This would potentially lead to a situation of just one company being in the game, which the FTC would not like. To a degree it is a matter of self preservation, the open communication between the two companies.