Bradley hasn't been very good this year. Still a good three-point shooter, but he's struggling from inside the arc (42% on twos, down from 50% the last two seasons), his defense is down, and Drummond negates his rebounding ability. He has become a floor spacer who provides little else. No wonder they want to move him.
Speaking of Drummond, a lot of people were concerned about Marcus Morris's inability to rebound at the PF position. He has quietly been a solid rebounder for us this year - TRB% of 12.0, up from 7.8. It appears that Drummond's presence just depressed his rebounding stats, just like what is happening to Bradley this year.