The rook on a8 was left to the queen
Hanging pieces and moved defenders 21...Nc5?? Black, game lost Opponent rated 906
Clock: this move took 4.1 s, and 1:47 were left after it.
At move 21 Black was clearly ahead (+6.8), but White's queen had just recaptured on d5 and now attacked the rook on a8, which had no defender. 21...Nc5 did not deal with it, and 22.Qxa8+ won the rook (−5.8).
Black resigned after 22.Qxa8+. 21...Re8 moves the rook and keeps the lead (+6.8). The knight move took 4.1 seconds, with 107.4 seconds left.
After every capture by the opponent, ask what the capturing piece attacks now.