PA is trending R because the dem trending areas have fewer people in them than the R trending areas, it's why PA moved from being D+5 in 2004 to D+3 in 2008 to D+1.5 in 2012 and R+2.8 in 2016, the state has trended Republican in 3 elections in a row and I expect that trend will continue.
If you look at the last Presidential election there were a little over 6 million votes in PA.
The below counties trended Democrat last time according to Atlas:
Chester = 270K
Delaware = 300K
Montgomery = 440K
Bucks = 345K
Lancaster = 244K
Allegheny = 657K
Butler = 98K
Cumberland = 123K
Centre = 77K
Technically Philly didn't trend Democratic last time but that was probably due to lower African American turnout. If you add that in:
Philly = 710K
If you add all the D trending counties + Philly, that's almost 3.3 million, which is more than half of voters. Plus the areas above are where pretty much all of the population growth is.