Aside from GC and a few other devs heading over to Riot, I think the main issue was pretty much that they massively under-estimated the work necessary for what they were trying to achieve with Garrisons in giving every single player custom instances. I think that's obvious from how far they ended up reigning things back from the ideas at Blizzcon, and how catastrophic the first few weeks were in particular around the Garrison instances continually crashing. That increasing investment in them is ultimately why they got carried away in making them so central and almost unavoidable content. They got too big and simply couldn't allow them to fail.
The further hardware investment because of the failings at launch will only have served to make them even a more critical development focus. I don't think its all that much different than Cataclysm's over-investment in rebuilding the old world sucking up too many resources. Throw in the extra hiring, and redistribution of staff from Titan's cancellation and there's easily enough upheaval and over-stretch there to cause significant delays. It doesn't need to be one specific catastrophe.