This has been discussed here before, but United cannot try to escape by citing the federal 30/60-seat rule, because United's contract of carriage states essentially that the same denied-boarding compensation scheme will apply to all of its flights, regardless of the number of seats on the plane. So the 30/60-seat rule is moot in this situation.

See http://content.united.com/ual/asset/COC24junfinal.pdf.

Additionally, I would argue that the language in the CoC used to define an IDB could apply to bumps due to weight & balance concerns, because the unused seats are not "available on that flight":

(Note, however, that there is an explicit exclusion in the case of a larger aircraft being substituted with a smaller one, which did not occur in OP's case.)

There are other apparent violations of the CoC in OP's case, as well. For instance, OP says that volunteers were not requested. In addition, I believe that an award ticket is treated as "full fare" for purposes of determining bump order, so the award passenger should not have been bumped. Of course, the GA could claim that getting the flight out on time trumped these considerations.