I think the reason is mental. I think his fame got to his head after he broke the WR.
After he broke the WR, in Uganda he was treated like a super star because it's a country which is not well represented in distance running, and there is a big competition between Uganda and Kenya. I think in one of the documentary Cheptegei explained that Kenyans tribes used to come to Uganda and kill people from others Ugandan tribe, and I think it happened to one of the tribe Cheptegei comes from.
So basic gist of it, Cheptegei was treated like a super-hero in Uganda. I think it got to his head and even if very subtle he just didn't train as purposefully as he was before. He lost this "fire".
Hopefully his recent bad performances will act as a cold shower, and wake him up.
I agree; he became overconfident and a bit arrogant after his year of WRs. Probably rested on his laurels instead of keeping at it with high intensity in training. I hope this is a wake up call for him. I was disappointed to read some of the arrogant statements he made in the last month or so. These revealed almost a lack of maturity, something I thought he had but maybe not. Not the type of talking we ever heard from the likes of Haile or Kenny, or Eliud.