Diagonal drag lines from hip to waist on the back of my trousers Fit Help

Drag lines point at what is tight. Yours run toward the centre back and there is pooling underneath, which is the classic signature of a back crotch curve that is too short and too shallow, so a full seat adjustment plus extra back rise.

On the pattern: draw a horizontal line across the back leg piece from the crotch point to the side seam, and a vertical line from the waist down through it. Slash and spread the horizontal line by 1.5 to 2.5cm at the centre back, tapering to nothing at the side seam, which lengthens and deepens the curve. Then add the same amount at the centre back waist and blend to nothing at the side, which is what stops the waistband dropping when you sit.

Do it in 1cm steps and re toile. This adjustment overshoots easily and then you get bagging, which looks similar and is more annoying to diagnose.

One more thing that gets missed: the shape of the back crotch curve, not just its length. Many patterns use a fairly straight, shallow back curve so they can be graded cheaply. If your body has a more pronounced curve, adding length alone does not do it, you also need to scoop the curve deeper by 5 to 8mm near the crotch point.

Before you commit, do the pinch test standing up. If you can pinch a fold of loose fabric under the seat and the drag lines relax when you pull the back waistband upward, it is short rise as described above. If pulling the waistband up makes it worse and the fabric was already loose, you have the opposite problem, a flat seat, and you need to remove length from the back crotch curve instead. Same symptom family, exactly opposite fix.

Also worth checking you did not grade the crotch depth when you graded the hip. That grade adds width but usually not rise, which is exactly how you end up here.