Given the 78 was a "divine inspired" move, and Demis tweeted that AlphaGo's mistake was move 79, my guess is that Black 78 was not in AlphaGo's search tree at all. It had pruned that move as being too "unlike what a human pro would do", and had not considered play beyond that point. Then it had to suddenly backtrack, drop its entire search tree, and start rebuilding from scratch. It took only a couple minutes to do so, but that's still not enough time to build up the depth and breadth of a tree that it had been constructing up to that point in the game. So it ended up playing a suboptimal move simply because Lee Sedol played a move that was simultaneously brilliant/strong and novel/unexpected, and it had to respond somehow.