Why wait till now? because A guy who has some clought [trump] is claiming it, and gaining traction.
[If born in Kenya] He probably didn't have the ability to forge it correctly as candidate. And hoped they could just paint people who were skeptical as crazy. [ad hominem, common tactic in politics]
Now they have someone [who is crazy,] standing up and shouting he was not born hear, louder than he can claim he was.