When I watch the video below I sensed that there's a deeper reason why Derrick Williams and DeMar DeRozan were guarding each other.
I remembered when Williams was still in his senior year in High School. He was heavily recruited by top Division 1 NCAA Schools such as University of Arizona and University of South Carolina (USC). He initially committed to play for DeMar DeRozan's school which was USC, but all of a sudden he changed his mind. He asked for a release from his letter of intent. He was later on seen donning the University of Arizona jersey. That I think is the deeper reason why DeMar tried to guard him one-on-one in their Drew League game last August 16, 2011.
DeMar probably wanted to send a personal message to Williams. But, unfortunately, due to Kobe's game winning shot, Derrick Williams' team beat DeMar's team, 139-137.
Here's the video showing Williams and DeRozan going at each other.