Juries don't do things like that unless they're pretty angry. My guess is that they thought one or more key witnesses for Simon's family firm lied under oath. That most commonly happens when there is a "smoking gun" in the documents. IMO someone on Simon's side of the counsel table screwed up big-time.
There are lots and lots of possible legal reasons for the trial judge to grant a new trial (my understanding is that he didn't reverse the verdict - he just called for a replay) without saying that the jury was wrong to be angry. This is not something for Simon's family to take lightly. They should take this as an opportunity to settle the case now that they know how badly they can be hurt.
As for the political effects, it's all spin at the moment. The damage to Simon's campaign has already been done. There is no possibility of this turning things around. Simon has to do things right in his campaign. He's running for Governor. His family's firm isn't.
Judge Chalfant awarded $125,000* to William E. Simon and Sons for legal/etc. costs, and he dismissed* the jury award. The ruling appears to be a reversal of the jury decision (to a lay person), although he did not actually "reverse" the award to make Hindelang pay millions to the defrauded* investors.
* facts from one of yesterday's AP stories here
See this thread for discussion: