A rook has as much trouble pinning an enemy rook as a bishop has pinning an enemy bishop—unless, that is, the enemy rook is pinned twice. Here White’s rook is pinned by Black’s. The White rook looks safe enough because it has protection from its queen, but this protection turns out to be White's undoing; it causes Black to ask whether the rook can be pinned to its guard as well as to its king. He plays Qd2 and now White’s rook is stuck on its square. Any move it makes will expose either White’s king or queen to capture. The rook is attacked twice and protected once, so Black will be able to bring it down next move with RxR+.

Your only worry is that White might be able to disrupt all this by replying to Black's Qd2 with Qd8+. We've seen this general risk before: a problem with pinning something to your opponent's queen is that the queen may be able to make a threat or capture that wrests away the initiative. In this case Qd8+ would force Black to move his king; and if White’s queen could safely keep the king on the run, it could unpin the rook on d3—and thus allow it to play RxR, since the rook at b3 would have been left loose. But once Black replies to Qd8+ with Kh7, White is out of options.