I'm a Penn State guy and a huge Hackenberg fan.
There was some regression of his own doing, but the OL was a huge issue.
http://www.ncaa.com/stats/football/fbs/current/team/468/p3
44 sacks allowed among the worst in the country.
There were 12 total scholarship OL on the roster last year, including 4 true freshmen who redshirted and didn't see the field.
Multi-year starter at G Miles Dieffenbach, now in Steelers camp as an UDFA, missed almost the entire season rehabbing an ACL.
That left a total of 7 guys as healthy options to play. Those 7 included Brian Gaia and Derek Dowrey, two guys who were at DT their first 2 seasons on campus before moving to the OL during spring ball last year because depth was so bad and two Redshirt Freshmen in Andrew Nelson and Brendan Mahon. Bottom line there wasn't a heck of a lot of talent/experience along the OL.
I'd look for a big bounce back year from Hackenberg with a stronger OL in place and all of his weapons outside returning.
My expectation is Bill O'Brien will do whatever is necessary next year to get in a position to take Hackenberg, they still speak regularly.