White has no loose pieces and Black’s queen has no checks. But we examine the consequences of every check as a matter of course, and here Black has two of them: NxB and Nf3. Nf3+ leads nowhere; White plays g2xN. But what happens after NxB+? White has to play QxN to recover his piece. That little exchange would mean a significant change in the position: Black’s knight and White’s bishop both would be gone, and White’s queen would be moved to e2. So rethink what would then be possible, noting any lines that would have been opened, any checks they would make possible, and any White pieces that would have been left loose. Black would be able to deliver a check with Qd4, and at the same time attack White’s newly loosened knight at c3: a perfect double attack, winning a piece.