Usually a pendant flips because the chain is too short for the wearer. We had a lot of customers complain their Perretti solitaire diamonds by the yard necklace would flip. (TIffany's) It would show the culet of the diamond and was uncomfortable. The solution was either an extender or the chain was replaced.
If it is a long pendant, it generally is because the pendant is too lightweight for the piece. Usually it is poorly executed and the entire design concept was poor.