The TPPA started as a deal between a variety of east Asian and Pacific nations in what the US regards as its back yard. The last thing the US wants is China becoming more cosy with US dependents such as New Zealand, Australia, or the Philippines. For the US, joining the deal, and then forcing it to either confirm to US wishes, or breaking it entirely are much, much better options than losing influence in the region.
Dependents? These countries have healthy trading with Asia and other parts of the world. From my perspective, given that China would eventually any trade hole left by the U.S., is that it comes down to cronyism.
The TPPA started as a deal between a variety of east Asian and Pacific nations in what the US regards as its back yard. The last thing the US wants is China becoming more cosy with US dependents such as New Zealand, Australia, or the Philippines. For the US, joining the deal, and then forcing it to either confirm to US wishes, or breaking it entirely are much, much better options than losing influence in the region.