Overwatch Experimental D.Va buff and Brigitte nerf was explained by Blizzard after the changes were a topic of conversation among the community. The latest update to the PTR brought several large hero changes, most notably a D.Va buff and Brigitte nerf.
Changes made in the experimental mode in Overwatch usually go live after a few weeks of testing, but can be stopped if the testing doesn't go well. These hero changes are no different and are not yet guaranteed.
Experimental D.Va Buff and Brigitte Nerf Explained by Blizzard
Brigitte got a big nerf in the form of no additional armor added to teammates by over-healing, shield regeneration rate was decreased, and the cooldown for the shield after being destroyed was nearly doubled. To balance those changes out the shield, health of the barrier was slightly increased.
In a post on the Overwatch PTR forum, Molly Fender explained the Brigitte changes were meant to balance her with the other support heroes. The test is focused on the removal of over-heal, and the barrier changes are only a secondary component.
D.Va's buff allows her to move faster while shooting, use Defense Matrix more frequently, and make the missiles launch faster. This is a considerable buff for the already mobile tank. Molly Fender says these changes were made to make the hero feel more "fluid and agile."