The author thinks that Lee Sedol was able "to force an all or nothing battle where AlphaGo’s accurate negotiating skills were largely irrelevant."
[...]
"Once White 78 was on the board, Black’s territory at the top collapsed in value."
[...]
"This was when things got weird. From 87 to 101 AlphaGo made a series of very bad moves."
"We’ve talked about AlphaGo’s ‘bad’ moves in the discussion of previous games, but this was not the same."
"In previous games, AlphaGo played ‘bad’ (slack) moves when it was already ahead. Human observers criticized these moves because there seemed to be no reason to play slackly, but AlphaGo had already calculated that these moves would lead to a safe win."
Which, I add, is something that human players also do: simplify the game and get home quickly with a win. We usually don't give up as much as AlphaGo (pride?), still it's not different.
"The bad moves AlphaGo played in game four were not at all like that. They were simply bad, and they ruined AlphaGo’s chances of recovering."
"They’re the kind of moves played by someone who forgets that their opponent also gets to respond with a move. Moves that trample over possibilities and damage one’s own position — achieving less than nothing."
And those moves unfortunately resemble what beginners play when they stubbornly cling to the hope of winning, because they don't realize the game is lost or because they didn't play enough games yet not to expect the opponent to make impossible mistakes. At pro level those mistakes are more than impossible.
Somebody asked an interesting question during the press conference about the effect of those kind of mistakes in the real world. You can hear it at https://youtu.be/yCALyQRN3hw?t=5h56m15s It's a couple of minutes because of the translation overhead.
