yeah i am going to read up on this guy Eric Cressey.
What i believe to be the cause of this is like you said overactive hip flexors( due to me sitting all day).
i used to to do sit ups on one of those sit up benches where the legs are secured between 2 pillow sort of things. and i used a lot of back in my deadlifts.
i have been strugling with this problem for quite a while now and i was told weak hamstrings was the cause, so i started with exstra hamstring work and now my back is the weakest muscle of those being used in deadlift.
as we know the dominating muscle in the movement will be worked hardest so that we can move the most weight, and since i probably have overactive hip flexors maybe thats the cause? all the ab work was removed from the movement.
and i have started doing " spider crunches " do you think those will give me strong abs while keeping the hip flexors out of the picture?