Jonathan Pie explains it

[DanaC]

Not the reason but almost certainly one of them.
The timing of his announcement clearly had the potential to impact early voting. It was a hugely damaging blow. The hardline hilary haters already considered her criminally corrupt - with that announcement those prepared to give her the benefit of the doubt were offered apparent evidence that there really is no smoke without fire - right before they were expected to cast their vote.
By the time the fire was revealed to be merely a smoke machine the damage was done.