Not really odd...a bunch of schools choose to hold White Coat at a different time other than orientation. NYCOM for example doesn't do white coat until before you begin rotations, unless that has changed since I interviewed there. Don't really think it has to do with being worried about people dropping out....the rate is very low during the first semester.
PCOM has a lot going on during orientation week, including student events during the weekend before you start class, so I would have personally found it inconvenient to have my family down when I was trying to get to know my classmates. Also, the ceremony occurs after what some would argue is the hardest test so it is something to look forward for. PCOM also sets up nice information sessions and demonstrations in the cadaver lab for your family and I think it is much nicer to have a grasp on the buildings and your professors before your family comes to visit.
All in all, it doesn't really matter when White Coat is...I wouldn't worry too much about it. You don't really need your coat at all in the beginning of first term, so it won't be an inconvenience.