Preliminary hearing don't last 1 full day let alone a week like this one did. It was the judges opinion based on what the Commonwealth presented that a prima facie case for the Involuntary Manslaughter charges didn't exist. He did however bound the other charges over to the Court of Common Pleas for trial. Only the judge can tell you why he didn't think all charges should have been bound over for trial.
I'm not sure how the DA would or could go about re-filing the involuntary manslaughter charge since other charges were bound over. I'm not in legal, I've been in law enforcement in Pa for almost 12 years and have seen how the court systems works from the District Court level up to the State Supreme Court.