For example
Say one class is doing really poorly in PvE but is doing great in PvP. New patch comes out. That class is getting a buff to its damage to make it stronger in PvE. There is no system in place to prevent that change from affecting its PvP performance, so now that class is godlike in PvP. The PvP balance squad now has to desperately try to make that class weaker and pray that the PvE team doesn't feel like buffing it any more.
There have been many real occasions where this happened.
Where is the synergy within Blizzard? Where's the communication between teams? It's as if the PvE and PvP teams are on separate sides of the country and are communicating with carrier pigeons.