Adam Schefter just tweeted the following regarding the Jammal Brown trade:
Trent Williams and Jammal Brown can play either OT position. But plan will be to start with Williams at LT and Brown at RT. Could change.
So maybe the Redskins were simply concerned with the right side of the line. I initially thought that they might have decided that Williams would be better off on the right side. However, for now it appears that the 'Skins just wanted someone other than Stephon Heyer or Artis Hicks at RT.