According to Myungwan Kim, 9p, commenting on the game in the AGA channel [1], Lee Sedol made a mistake right at the beginning and spent the rest of the game hoping to claw it back; he said he would resign about 15 min before he did.

EDIT: but this postmortem [2] of the game is far more nuanced and doesn't reach the same conclusion.

[1] https://www.youtube.com/watch?v=6ZugVil2v4w [2] https://gogameguru.com/alphago-defeats-lee-sedol-game-1/