Several possible reasons apart from the slide in the PoO, and that of gas which has affected the SP of all O&G companies.
A perception that PSA havn't replaced reserves in recent times as fields deplete.
Uncertainty around Beibu Gulf development.
Effect of recent hurricane in Gulf of Mexico - several facilities shut in as at 18 September.
Biggest reason may be that announcement of 18 Sept said that there would be an update in a week's time. Hasn't happened!

A perception that PSA havn't replaced reserves in recent times as fields deplete.
Uncertainty around Beibu Gulf development.
Effect of recent hurricane in Gulf of Mexico - several facilities shut in as at 18 September.
Biggest reason may be that announcement of 18 Sept said that there would be an update in a week's time. Hasn't happened!