Since Patrick Mahomes took over as their starting quarterback, the Kansas City Chiefs have never had a losing record. Until now.

After three weeks, the Chiefs are looking at their first 1-2 start since 2015 and they’re alone at the bottom of the AFC West. Their loss to the Los Angeles Chargers on Sunday marked just the fifth time Kansas City had lost to their division rival in the 17 matchups since Andy Reid took over as head coach. So, what happened?

We’ve really started to see some of the holes exposed on the Kansas City roster, especially on the defensive side. In their win against the Cleveland Browns, the Chiefs seemed incapable of stopping Cleveland’s offense. They allowed Baker Mayfield to lead four touchdown drives of 75 yards or longer, including two fourth-down conversions in the Chiefs’ red zone on consecutive drives to keep their scoring hopes alive. Against the Chargers, Kansas City allowed another fourth-down conversion just outside their own red zone, plus a costly pass interference penalty on a late-game fourth down allowed Los Angeles to keep their game-winning touchdown drive alive.

The Chiefs’ red zone defense has been awful this year, and not just on fourth down. Through three weeks, opponents have scored touchdowns on 12 of 13 trips to the Kansas City red zone. No team in the NFL has given up as many touchdowns in the red zone per game as the Chiefs (4.0)—almost a whole extra red zone touchdown per game than the next worst team in that category: the Green Bay Packers with 3.3.

The issues have not just been in the red zone, either. The Chiefs have given up the most yards per play, the second-most rushing yards per game, and the ninth-most passing yards per game of any team in the NFL. Their run defense hasn’t been able to set the edge to contain outside runs, they haven’t been able to shed blocks, and they’ve been really bad at finishing tackles.

On the passing side, the Chiefs’ decision to move defensive tackle Chris Jones to defensive end hasn’t paid off at all in the pass rush department. He finished Sunday with just one hurry and one tackle for a loss out of two total tackles. On the other side, Frank Clark has played in one game and had zero pressures with zero hurries and a single tackle for a loss. Kansas City’s sack percentage is bottom five in the NFL this year and they’re tied for the least QB pressures in the league through three weeks.

Of course, when you’re looking to avoid watching your defense get exposed drive after drive, it helps to avoid turning over the football. The Chiefs haven’t been good at that, either.

Kansas City is averaging one fumble and one interception per game, and each turnover has felt more costly than the last. First, the Baltimore Ravens intercepted Mahomes—who had never thrown an interception in September—on a desperation toss-up to avoid taking a sack that led to a Ravens score late in the third quarter. Later that game, running back Clyde Edwards-Helaire’s first-ever NFL fumble sealed the win for Baltimore. On Sunday, Mahomes overthrew tight end Travis Kelce on a throw off his back foot with less than two minutes left in a tie game. It gave the Chargers great field position and Los Angeles was able to finish off their game-winning drive with a late touchdown.

Bringing up all of those turnovers and Kansas City’s poor defensive play isn’t to say that the Chargers don’t deserve any credit for their win. For one team to turn the ball over, their opponent usually has to force one. To that end, the first three Chiefs possessions ended with a turnover—the second interception of rookie Asante Samuel Jr.’s career followed by two fumbles, both forced by Tevaughn Campbell. Secondly, not every team can take advantage of a poor defense, but that’s just what Justin Herbert did, adding four touchdowns to his season total on Sunday.

Another big part of the Chargers’ success was limiting the Chiefs’ explosive plays. Kansas City only had two completions over 20 yards on Sunday and their third-biggest play was a Mahomes 21-yard scramble. And while part of it has to do with Kansas City lacking a WR2 upgrade after Sammy Watkins’ departure, you’ve got to give credit to the Los Angeles (and Baltimore) secondary for locking down Tyreek Hill, who finished with 56 receiving yards and no touchdowns in Week 3. In all, the L.A. secondary held Mahomes to 5.91 yards per attempt, which is the third-lowest mark in any game of his career.

If the Chiefs’ defense continues to struggle while opposing teams figure out ways to shut down Hill, Kansas City will have a hard time convincing people they’re built for a third straight Super Bowl run.