According to the South China Morning Post (Hong Kong) Boeing had issued worldwide alert after two similar cases of the bolts coming off but did not issue a mandatory change of the bolts. It said that China Airlines underwent checks after the alert. There are lot of high speed airflows around control surfaces that would impart a lot of energy to lose bits of metal but the question remains of how do bolts, tightened and then subsequently rechecked, come loose enough to fall off in the first place. Design flaw?