When an ability buffs all damage dealt by a player, the game is coded to actually provide a series of 16 different modifiers to the player, each of which increases damage of a specific type by an appropriate amount. If the ability stacks, each stack applies all of those modifications separately. The result was that the game’s processes for identifying and processing stat modifications were getting overloaded, and the team is going to roll out a new setup to group these modifications together, which should reduce load, improve performance, and be totally invisible to players. Except, of course, for players who have read about it.
Source: Reddit; thanks to ThatLantestGuy for the tip!