The video changed rumors of ill health into a reality for all to see.
I would say the big three were:
Continuing email server controversy.
Stumbling video.
Wiki DNC & Podesta emails.
She might also be a drunk. Her 9/11 condition looked as much like drunk as it did Parkinsons. And, the email points that direction.
I think the “deplorables” comment hurt her as well. It showed her true nature and gave Trump supporters a rallying cry.