It's no secret that the Chiefs defense did not play particularly well against the run in 2014. All one has to do is take a look at the numbers to see it (but don't do it, because they're ugly and I want you to have a nice day).
There have been a lot of conversations about the CAUSE of this struggle since the season ended. Obviously, the absence of Derrick Johnson and Mike DeVito hurt, as did the absence of Eric Berry. When your defense loses three of its best run defenders it's going to suffer a little bit at least.