Well, one could assume they are only trading down after they realized their guy was gone. Picks in themsleves are not valuable. If the Texans believed one of those WRs was going to be a pro-bowl WR then you sit in the spot and take him...but if they are gone and you were set on those guys then you trade back. Out maneuvered in that it's very possible that the Texans thought they were getting one of those WRs and that didn't happen when two teams traded up ahead of them...which could mean they knew who we were targeting. All of this is guessing of course, I don't work in the Texans FO.
So you trade back two spots and one of Hamilton or Johnson are guaranteed to be there. That's pretty damn good value if ultimately those were guys the Texans were targeting all along.
I know this is a bad comparison, because the rest of the defense was pretty stacked, but I'll always remember that one draft when they took 4 DBs in the 1st 4 picks and then another one later. Changed their defense.
More than likely Eagles got jumped and we took advantage. That trade happened the same time the Detroit trade happened
My only concern about building around a secondary is that the NFL doesn't really let players in the secondary play any defense these days.