1. Those who resent the way the ruling party reneged on its promise of legislative review and forced a this trade services pact with China through.
2. Those who have economic concerns, e.g. they worry that Taiwanese small and medium sized businesses would be acquired by Chinese capital thus massively raising the barrier to entry in some industries. Another worry is that Taiwan may face a massive brain drain to China.
3. Those who have national security concerns because the trade services pact opens for e.g. some parts of the telecommunications and publishing industries. China claims Taiwan as a renegade province and has said it'd use force to unify the motherland if necessary.
So far this is still a Taiwanese domestic issue though with the U.S. and China watching closely. One WSJ report goes[1]:
"U.S. officials might consider all this as they prepare to resume bilateral trade talks with Taiwan next week. Advancing bilateral trade—and encouraging eventual Taiwanese accession to the Trans-Pacific Partnership—would help quiet nerves on all sides of Taiwanese politics. The U.S. has largely ignored Taiwan in recent years, but Taipei's current crisis highlights the extent to which trouble still lurks in that corner of Asia."
[1]:http://online.wsj.com/news/articles/SB1000142405270230441840...