It's both the WRs and Cassel, plus some fault to the playcalling. Bowe drops a lot of balls (Chambers doesn't though), but Cassel throws the ball right into traffic enough that some of the drops are his fault.
And they run the routes that the OC calls. They aren't making up their own routes out there. Cassel doesn't have the arm or the accuracy for the long ball right now, so Weis is gameplanning around that. And if Weis calls another pitch to the RB on 3rd and 1, we need to fire him. Out of a cannon and into the sun.