According to the answer key of a SAT preparation book I'm studying, there is an error in the sentence
By the time the bank guard closed the doors, a riot had erupted due to the long lines and shortage of tellers.
and it should be changed to
By the time the bank guard closed the doors, a riot had erupted as a result of the long lines and shortage of tellers.
I disagree with this. Why is the first sentence unacceptable?