Think visually. White’s king and a6 rook are on the same diagonal, inviting Bc4+. Or again you could see this by asking what checks Black can give with his bishop and seeing that one of them—Bc4—also attacks a rook. Either way the problem is that White protects c4 with his other rook. There is no immediate way to get rid of the rook, but before moving on ask what happens if Black goes through with the fork and White plays RxB. What checks could Black then play? Rb8-b1, which is checkmate (after White uselessly interposes his rook from c4 to c1). So Bc4+ wins a rook despite the apparent protection of c4 by White’s rook, which really can’t afford to move. The tipoff here is Black’s other rook on the second rank; it traps White’s king on the first rank, making it vulnerable to a back-rank mate if the rook on c1 leaves its post.