Someone correct me if I'm wrong, but I don't believe that Comey publicly announced that the investigation was reopened. I believe that he informed Congress that there was new information available, as he had promised to do if the investigation reopened, and it was republican members of congress who actually broadcast it publicly. My take-away at the time was that it was the members of congress who behaved horribly unethically, not Comey.
I'll be glad to be corrected if I'm wrong. I don't have time to research it right now.
I remember that Comey was faced with the very real prospect that the New York FBI office would leak the fact that many more emails had been found. Support for Trump was high in the FBI and there had been many leaks of information detrimental to Clinton, especially from the New York FBI office. This was more than a little unusual, leaks are normally rare from the FBI's investigations. It is a breakdown in their professionalism.
Therefore Comey was faced with only two choices, and both of them were bad. Sit on the information and to suffer when the almost inevitable leak from the New York field office exposed it and exposed Comey's failure to inform Congress as he had promised, or to inform Congress, with the certain knowledge that it would be leaked by Congress in a heartbeat. He decided to inform Congress as he had promised and to avoid the blow back and damage to the FBI's reputation that would accompany a leak of this size from the normally taciturn FBI.