It played a well known selective game and did whatever was possible to prevent a certain players from getting a precious south bridge, it is alleged. Nvidia sees ATI as a serious chipset competitor as ATI ships more chipsets than the green boys but Nvidia is still most popular one.
If there was such a strategy, such a strategy worked as many players were late to market and missed the RD580 + ULI 1575 wave. Now they are all fine as ATI is getting ready to ship its long awaited SB600 south bridge and its partners won't have to go to ULI to get a south bridge.
This is called competition.