The 5102 cannot have more pretrigger points than it has onboard buffer memory - 663,000 bytes. By default, the trigger position is 50%, or halfway through the buffer. Taking 8 million points/channel on both channels allows you 331,500 points/channel or a maximum trigger position of about 4%.
Set the trigger position using the reference position input of niScope Configure Horizontal Timing.vi. If you want to always be safe, use zero.