Blizzard released Starcraft in 1998. One of the most popular custom maps made for it was Aeon of Strife, a player controlled map where you fought with a hero against AI-controlled enemies in three lanes. This particular map had a large player base and later on, year 2002 to be exact, it evolved into DotA when Blizzard released their Map Editor for Warcraft 3. This was arguably the beginning of the MOBA genre which over time became one of the largest topics in the gaming industry.
Both these custom made maps were popular, but Blizzard never took a step towards monetizing the genre that was, at that point, entirely based on their platform.
Blizzard announced World of Warcraft in 2001 followed by a closed Alpha testing phase. This client was later on leaked, datamined and analyzed by a dozen of people. Even an emulator was released, it can still be found on this site. Much later on Blizzard announced the 4th expansion Mists of Pandaria and stated it would be coming with a MOBA style battleground. When the patch 5.2 hit live, a map titled Defense of Alehouse was found. It was not unclear at this point that this was the rumored battleground.
Now let's compare these two minimaps: First being Kalidar, an unused map from the Alpha client (0.5.3), and the other Defense of the Alehouse from WoW 5.2:
These look quite similar, and actually, they overlap each other almost perfectly. Even though the three-lane mindset is rather common in Blizzard design, you can't really miss the similarity not only between the map structures but also with the design philosophies.
What I find really mind-boggling is that the upcoming Blizzard MOBA Heroes of the Storm has this unique feature of allowing people to access the mines, a level which enables players to move from one side of the map to another via an underground passage. Somehow, this in some sense seems to apply to Kalidar as well it having two mines on both sides that print out a message like this when entered:
Could this all be a coincidence? Perhaps Blizzard saw the potential in Aoen of Strife very early on, but weren't really in forced to really start developing a MOBA of their own until they saw the industry go nuts with all these big names like Dota 2 and League of Legends taking up their playerbase away from WoW.
This is just some speculation I came up with. The evidence I gathered is rather minor, but nonetheless, I hope this thread was an interesting read.