What is the reason for bulging veins in the booty?

Buttock veins. Buttock veins are not the routine for location of varicose veins and when they occur, sources for them such as the saphenous vein, branches of the saphenous vein or even reflux from pelvic and ovarian veins need to be considered. A venous duplex ultrasound of the leg veins may help and, sometimes, a CT scan of the pelvis may be necessary. See a vein specialist.
Hemorrhoids? . I wonder if you are asking about hemorrhoids which are most often caused by straining to pass stool. Avoiding constipation and over excessive straining on the toilet.