What checks does Black’s bishop have? Bd4. Look to see what else the move does and note that it attacks the loose rook at a7. So there is a potential bishop fork at d4—but the square is protected by the knight at b3. So of course you look to take the knight and thus play RxN. The response a2xR follows, and after the exchange d4 has become a loose square. Black safely plays Bd4+, White’s king moves, and Black takes the rook. All this is familiar from our prior work on double attacks. But notice the different visual look of this position; see the White rook and king stretched out along the same diagonal, and realize that this signals a forking possibility.
As discussed earlier, a rook often can make a good target for a bishop fork whether it is protected or not. Here, however, the fork worked only because the rook was unprotected. The reason, which is common, is that the fork required a sacrifice by Black in the first place—here, a sacrifice of his rook for a knight. To justify that sacrifice, the fork that it creates has to make a big score. Taking a rook for nothing makes it worthwhile. Taking a rook that had protection, and then losing the bishop, would have made the combination a wash.