Why placental abruption happen?

i lost my son last nov.16 2008 at 38weeks due to placental abruption..on that morning i feel like menstrual cramps but not so hard and painfull and no l bleeding i thought that sign of labor at 4pm more pain and run to the hospital check my cervix with 3cm dilated after that i bleed frsh red blood and more painfull like menstrual pains but when my OB check my babys heartbeat no beating my son was died and had my emergency CS after 2days i saw my son hes died its very hard to me whats happen to my baby why he died complete prenatal check ups during pregnancy..what happen? any thoughts pls?