MLB Umpire Crews

[johnSandlin]

As anybody noticed lately all of the shifting around with the MLB umpiring crews. Poncino went over from Darling's crew to work with Mike Reilly's crew.
I know they are trying to get as many umpires evaluated as possible, and also trying to work as many of the AAA guys as possible, but it seems more switching is going this year with the crews.
Can anybody offer any other insight as what the reasons might be for all the switching around of the crews?