Everything happens too quickly for Aragorn to think; he can only react.
One moment he and Boromir are tugging frantically on the chain that confines the cave troll. Even their combined strength may not be enough to pull the creature away from Sam, and Aragorn cries out wordlessly as he struggles.
In the next instant, Sam has escaped, the troll has turned, and the wrist that Boromir has wrapped through the loops of metal to secure his grip has become ensnared. He is caught as though he and not the troll were meant to be held prisoner by the chain.
Before Aragorn can move, Boromir flies across the chamber far above the floor, slamming into the distant wall with a sickening thud. Sliding downward, he crashes against his shield on the high ledge that circles the chamber. His head strikes stone. His body does not move as he tumbles from the ledge to the floor, where he lies unmoving.
A single, terrifying instant passes and already the orcs begin to close in. In the noise of the battle, Aragorn cannot hope to shout Boromir's name loudly enough to bring him back to awareness. Nor can he reach the other man's side quickly enough to protect him. The passing time grants Aragorn only recognition of how afraid he is and how badly he wants to save the Steward's heir.
Suddenly Boromir shakes his head, trying to toss the hair from his eyes. He starts to sit up but an orc looms over him with a knife poised to stab him. Boromir does not recoil from the blade, just as he did not flinch from the cave troll. He meets the eyes of his attacker steadily, and the orc hoots in triumph, preparing to strike.
In those moments, Aragorn seizes his nearest weapon and hurls it. The sharp point slices through the orc's skull, killing it instantly, making it crumble to a floor already littered with bodies and filth.
Boromir's eyes reverse the flight of the blade, following its path to meet Aragorn's gaze. For the first time Boromir looks momentarily startled, as though he cannot grasp why in the midst of the battle Aragorn has chosen to defend him rather than the Ringbearer.
At once Aragorn is glad that there is no time to talk, nor even to think, and that the noise of the fray would bury any words he might speak. He nods once at Boromir -- a gesture meant both to reassure the other man that he is well and to forestall any need for expressions of gratitude. Then he turns, heart pounding with the brutality of the fight, leaving Boromir to defend himself.
Now he knows that he faces more than one conflict; now he knows that he must redouble his guard.