Here's the problem: Obama has made the US so unreliable and unpredictable that the Phillipine government has no idea (or faith) that he will support them in anything. And there they sit with "disputes" with giant China. China will gobble them up if they want to and the USA will probably (under Obama) do nothing but give some speech about how mean they are and how a sternly written letter is on the way to the UN right now.
They decided to align with China now when they had some ability to negotiate the terms rather than wait for the inevitable.
Bottom line: Trump is right when he says nobody respects (or trusts) Obama / Clinton.
Obama believes US influence is malignant and all his policies flow from that point.
Trump's position is that we'll protect countries if they pay us, so the Philippines are kind of hosed no mater who wins. But if he really think he's going to get any kind of accommodation from the Chinese then Duterte is nuts.