I get this question, fairly often "I just merged from Trunk -> Branch, resolved all conflicts and checked in. I now merge from Branch -> Trunk, and there are no changes in trunk, so I expect no conflicts, but I get conflicts - why?". The short answer is you will get conflicts on any files you had conflicts on in the merge from Trunk -> Branch and resolved as KeepYours or AutoMerge. The reason for this is that different branches have different purposes, so a resolution of a conflict in 1 direction, doesn't automatically imply the same resolution in the reverse direction. Consider the following scenario:
Fixed Bug 1 + refactored code
Fixed bug 2
Merge from Development, discarded refactoring took bug fix
In changeset 50, you accepted a bug fix from Development, but discarded a refactoring which was not needed for servicing. Now you want to push back change 40 to the Development branch. In this case, you don't want to blindly accept the Servicing version of the file, hence we present you with the opportunity to choose content coming back to Development. TFS mechanism of doing this is filing a conflict. The same reasoning is applied for the KeepYours case.
Hope that helps explain things, when you do hit this scenario.