almost always falls to the ground immediately, as the structure supporting them has just been broken.
Care to explain to us exactly what has broken and why that has resulted in the person going to the ground?
The problem is, most people don't know where to actually aim to cause a pelvic collapse that results in a locomotor collapse. Sure, a lot of people shot there will go to the ground. The same is true for a lot of people shot in the gut or chest.
At the top of the page is a view of a gunshot pelvis xray. The majority of what you see in the image that is bone is comprised of the iliac blades and sacrum. Not shown as it would be below the image is where the femur articulates with the pelvis, where the femoral head fits into the acetabulum (pelvic socket for the femur).
The innominate of the pelvis is comprised of three bones, the ilium, ischium, and pubis. You have an innominate on each side. The largest is the ilium. It is what you feel on your, or as your hip. It is so large as it helps support the contents of the abdomen as a hard support. It can be penetrated or even fairly sections broken off without causing a mechanical incapacitation of the locomotor activities - not disrupting the ability of the femur to mechanically be able to function with the pelvis, and without the pelvic circle being broken and allowing the pelvis to 'collapse.'
I like this xray as it shows that the bullet passed through a section of pelvis, the iliac blade and did so in an area not directly locomotor.
I believe if you check with the National Museum of Health and Medicine, you can be provided with some images of battle damaged pelvic and femoral bones and see that indeed they can be struck without causing a locomotor cessation.
The bottom line is that rumors of broken or collapsed pelvii are greatly exaggerated. People may go down when shot, but not for the reasons believed.