All the reports from the team said Hillman was sick. But let's run with the conspiracy theory because that's more fun.
I'm normally not one who puts much stock in rumors and innuendo, but I think there's a decent chance this one has some legs to it. Apparently Ronnie Hillman looked plenty healthy the week leading up to the game against Indy, so I'm skeptical that his stint on the inactive roster had anything to do with any lingering health concerns. I think the "Ronnie Hillman and Kayvon Webster did something they weren't supposed to in Dallas, had to go to the hospital because of it, and were demoted early in the season as a punishment" story fits pretty well with the facts on the field.
Either way, the simple truth is that Ronnie Hillman got to rest on week 4 of the preseason while C.J. Anderson had to play. Whether Hillman missed week 1 and was glued to the bench in week 2 because he was being punished or because he was feeling the lingering effects of whatever put him in the hospital, Hillman was pretty clearly ahead of Anderson on the depth chart before that, and he's pretty clearly been back ahead of Anderson on the depth chart since, outsnapping Anderson 14-3 against Seattle and playing well against Arizona while Anderson was inactive. If I had to bet on a Denver back in the short term, that'd be my bet. At least for now. We've seen Hillman blow opportunities plenty of times before this. I have very little faith that this time will finally be the time he makes the most of his chance.
If I were placing a bet on what happens long term? I know everyone is eager to push him out the door, but I think it's going to be Ball's job again whenever he gets healthy. Sort of like Ben Tate in Cleveland, or Rashad Jennings in New York, or Knowshon Moreno in Miami, (or Knowshon Moreno last year in Denver). Everyone is eager to push those boring incumbents out the door for the next big thing, but NFL teams keep going back to them for a reason. Who knows if Denver will get its running game sorted out enough for the lead back to have any fantasy value, (I think they will, but they certainly haven't so far), but if any back is fantasy relevant over the final 6-8 weeks of the season, I think it's going to be Ball.