>>149232692she's sitting on a bed which is deforming her butt.
look hyper anatomy is a system and if the whole system isnt set up then the parts look out of place. the pose and position and cropping werent right. id be better off drawing it from scratch.
lastly, yeah actually butts are square. your coxis and pelvic ridge meet at a 90 degree angle and the glutes are attached to the great trocantor on the femur which makes the 4th corner.