Making the same error ripping boards

I’ve been trying to rip a 2X4 to make a quick saw vice. The first cut went perfectly, over two feet of rip staying less than 1/8 of an inch from my gauge line, allowing me to quickly plane down to the line. Then I tried the cut in the middle to fit the saw plate into.

And I keep making the same mistake and I can’t figure out what it is. Sooner or later the cut starts to turn to the Left. I’m Left handed. And it doesn’t just take on an angle, it cuts a curve as the photo should show. This last board I didn’t try to correct, I just kept cutting, paying careful attention to my body position but otherwise not fighting the saw. I’ve tried dropping the saw hand, cutting one side of the board, then turning the board around to cut the other side, etc. I get the same result.

As you can see, the cut starts off looking quite straight. Then it starts to pull away, creating a 5/16″ variance in the first 11″. Over the next 11″ that becomes a 1 11/32″ (one and eleven thirty seconds inches) creating not just an angle but a curve with a flat panel saw.

Interestingly I’ve TRIED to cut curves and to reverse the curve with this saw but have been unable to do either.

Because the first cut went perfectly I’m confident it’s something I’m doing. I’m keeping my saw arm swinging cleanly and the plate isn’t binding in the cut much but I’m quite consistently getting these curves.

HOW am I doing this???

Attempting to add multiple pictures so you can see what I’ve done.

Part of the reason I’m doing this project is for the practice in ripping but at this point I feel I’m practicing my errors.

Any insight appreciated.