In a word? Philadelphia!
I bet Casey is ahead in Philadelphia by 110%.
"In a word? Philadelphia!"
In another two words: voter fraud (in Philadelphia).
[FYI: President Bush lost PA by a mere 144,248 votes; he lost Philadelphia by 412,106 votes -- remove Philadelphia from the equation and the President (Santorum) wins PA easily!!]
Lots of excellent 2004 PA election data at
http://www.uselectionatlas.org/USPRESIDENT/state.php?year=2004&fips=42&f=0