This defense has some major issues that hadn't been exposed until recently. For me, it all starts with the defensive line, particularly the tackles. The aren't doing a very good job, either stuffing the run or preventing the qb from stepping up when passing. I think we are seeing the results of recent misses in recruiting. Because our front 4 don't get enough pressure/stop the run, it puts pressure on everyone else. Guys like Webb, Arnette, and Worley are getting exposed because plays are lasting way too long.
Even if they don't get sacks, pressure (as we saw with Joey Bosa) can get qbs taking their eyes off of downfield, forces them into making mistakes, etc.
I also think the defensive coaching has been horrendous. Against Penn St, we gave up a TD at the end of the first half because we were playing man coverage. Who plays man with under a minute left in the half? That was inexcusable. How many times today did you watch the same two types of plays: wide receiver out, TE crossing. They did it play after play after play. This coaching staff, on both sides of the ball, do the players no favors.