What was the main difference in the team's defense between the 1st and 2nd half of the season?
Uneducated guess:
The individual players became more comfortable with their responsibilities. In addition, they began to trust the schemes more which allowed them to completely focus only on their job.
Other things:
1. We had a new DC come in and he had no time to prepare the rookies or veterans in the off-season for a completely foreign vernacular and philosophy.
2. We had a ton of young talent step in (Jefferson, Carter, Johnson after injury, Peterson, etc) and they needed time to mesh with established players.
3. Horton may have overwhelmed the defense in the beginning with more than they could handle. This lengthened the learning curve and is probably typical of new coordinators.
4. Our offense wasn't really great so they were playing a ton of snaps. And it was compounded by the lack of success defending on 3rd down.