Could it just be excess fat? For example the animal may have been highly obese previously and when the orangutan has lost weight has left fatty deposits behind. A great exmaple was a mongoose lemur at Bristol zoo, which had like a big ball dangling form its chest.
It is very unusual. It isn't pregnancy- as you said that gives a tight, rounded abdomen. Foz's suggestion of sudden massive weight loss in a formerly obese animal is a good one- though I've never seen it in an Orangutan before. Another suggestion might be some sort of tumour or abnormal skin growth?
The only way you will find out for sure is asking the Zoo people- I'd be interested in the answer.