What does they refer to in each of the following two sentences: 
a) The authorities have denied authorization to the demonstrators because they feared violence.
b) The authorities have denied authorization to the demonstrators because they appeared belligerent.
Syntactically speaking, in both sentences they could refer back to either the authorities or the demonstrators as the antecedent. However, based on the semantics of the sentence, there is a more likely antecedent in either case. In sentence (a), they likely refers to the authorities, because it makes little sense for the demonstrators to fear violence and for that to cause the authorities to deny authorization. In sentence (b), on the other hand, they probably refers to the demonstrators, because apparently belligerent demonstrators are a typical reason to deny authorization, and apparently belligerent authorities are not.