Adam Schefter of ESPN reported that Von Miller was facing a four game suspension for violating league policy, but he hasn't given us a description of exactly what Miller has done wrong to warrant such a suspension. Now, more details are emerging from other reports.
Mike Klis of the Denver post is reporting that Von Miller violated the league's substance abuse policy and the league's PED policy. Klis reports that Miller tested positive for amphetamines and marijuana in 2011. Since the marijuana violation would take at least three such violations to trigger a suspension, it's safe to assume that the amphetamine test, which falls under the league's PED policy, is the reason for the suspension, assuming the report is accurate.
Miller will, or more likely has, appeal the suspensions, and that appeal will take some time, meaning this situation is far from over. Until it is resolved, the Broncos will not know exactly when the suspension will take place, meaning it could be more beneficial for Miller to actually just get the suspension out of the way early in the season.
For Miller, the report damages an otherwise solid reputation. Even if he wins his appeal and avoids a suspension, in the public's eyes, he's a drug user, and he'll have to be more careful in public actions. The league may eventually back off a suspension, but once such information is made public, there's no taking it back.