Overuse of prescription pain killers (or opiates like morphine) will cause constipation.
"We just assumed that the constipation was secondary to the meds that he was taking for his arthritic pain and for his insomnia.”
((snip))
"But bowel paralysis is hereditary and you can in fact pass it down to your children,” he continued. “His condition was either something he was born with like Hershberger’s disease, or some viruses cause the paralysis of the nerves in the colon. The nerves weren’t functioning enough in places, or weren’t functioning at all because his colon would not push food out, it would just accumulate.”
The article says that's exactly what they thought at the time. But the autopsy reportedly revealed it was more serious than they thought.
All joking aside, the article makes a number of interesting points. Elvis's pride prevented him from getting the treatment he needed for this condition: a colostomy (that was the standard treatment in the 70's).