Clowney not being on the list is significant, as is Cushing being on the list.
At this point in the year, PUP is mostly a procedural designation because a player can be taken off it at any point before the first week of the regular season. If he is still on PUP during Week 1, then he must remain there for at least six weeks.
There is clearly no concern, as expected, that Clowney will miss any of the regular season. In fact, this indicates the Texans aren't expecting him to miss much time, if any at all. Clowney had surgery to repair a sports hernia in June.
In Cushing's case, this means the Texans are preparing for the possibility that he could need to start the season on PUP. In order to be able to put a player on regular-season PUP, he must start there before training camp. Cushing is certain he'll be able to play all 16 games this season, but the fact that the Texans apparently think there's a chance he won't be ready for Week 1 is noteworthy. Either that, or they're playing games.
Williams spent last season on injured reserve with what was termed a knee injury. He was a fourth-round pick in 2013. Nix was a third-round pick this year and worked through a knee injury during the offseason.