After popping in some tape to discover the reasons behind the sudden change, here is what I saw:
Defense has been adaptable
The loss of safety Eric Berry robbed the Chiefs of their most important defender. Defensive coordinator Romeo Crennel took full advantage of the second-year man's athleticism and versatility. Berry would cover receivers and tight ends in the slot and creep near the line of scrimmage to become an extra defender against the run. His ability to seamlessly switch roles was a vital part of the Chiefs' game plan and it took several weeks for Crennel to figure out how to compensate for the loss.
Crennel inserted veteran safety Jon McGraw into the starting lineup and he has responded in a big way. As the hybrid safety in the back end, he floats between the deep middle and box area based on the opponent's tendencies. Although he lacks the athleticism and explosiveness of Berry, his savvy allows him to deliver big plays.
Crennel has also been willing to use more sub-packages to match up with opposing personnel. This has meant more man-to-man coverage and gimmick defenses to neutralize the opponent's top weapon. For instance, he frequently used double coverage against the Chargers' Antonio Gates, aligning two defenders side by side over Gates to discourage throws in his direction, while also challenging the Chargers' other weapons -- Vincent Jackson and Malcom Floyd -- with aggressive press coverage. Although the Chiefs surrendered 369 passing yards, they kept the Chargers' explosive trio out of the end zone and frustrated Philip Rivers into a sub-par performance.
Crennel displayed a different approach against the Raiders a week earlier when he instructed his cornerbacks to utilize more pre-snap disguises to take advantage of Kyle Boller's inexperience and Carson Palmer's rusty game. With both quarterbacks unable to identify coverage on the fly, the Chiefs forced six interceptions on the way to shutting out their division rivals.
Johnson, Hali playing at All-Pro level
Johnson, the team's leading tackler, has recorded 10-plus tackles in consecutive weeks. He made a pivotal fourth-down stop of Michael Bush at the goal line against Oakland and delivered a red-zone interception of Philip Rivers against San Diego. As a result, the Chiefs have only surrendered touchdowns on 4 of 11 red-zone possessions during their four-game winning streak.
Hali has been an absolute terror off the edge. Although his six sacks fall well short of the numbers set by the league's sack leaders, he has posted multiple sacks in two of the past four games, including a two-sack effort against the Chargers, and forced numerous penalties (false starts and holding) due to his pass-rushing prowess. He has been exceptional in the confines of Arrowhead Stadium, where his first-step quickness has been enhanced by the deafening noise of the home crowd. With offensive tackles unable to hear the snap count, Hali has routinely sprinted past blockers on the way to recording five of his six sacks at home.
Crennel has taken advantage of his star rusher's dominance by dropping seven and eight defenders into coverage in passing situations. As result, the Chiefs have held opponents to a 35.8-percent conversion rate on third down during the four-game streak, which is significantly less than the 46.5 percent allowed during their first three games.