Marketing missteps (perhaps the most similar, in WotC's favor there isn't a large competitor ready to pounce on their missteps)
With New Coke, the main competitors were Pepsi and, most relevant, the original "Classic" Coke.
While there is no game that could be considered the "Pepsi" to WotC's 4Ed "New Coke," 3.X in all of its variants (including 3rd party versions like AU/AE, Pathfinder, and Midnight) are definitely analogous to "Classic Coke"- namely, highly successful products with a strong and vocal following.