The king walked in

46.Kb6 Rb8+

White’s king marches up the board to hunt pawns. Black’s queen and rook are waiting.

46.Kb6 Rb8+
The walk. 46.Kb6 Rb8+. The king is on the queenside and the rook checks it.
47.Kc7 Qa7+
Deeper. 47.Kc7 Qa7+.
48.Kxc6?
One pawn too many. 48.Kxc6. The king grabs c6.
48...Qb7#
Mate. 48…Qb7#. Queen backed by the b8 rook.

Lesson: With queens and rooks still on, your king hunts pawns only after the enemy checks run out.

https://www.chess.com/game/141266113778