Defence has a shape
When a fighter defends a triangle choke, an armbar or a rear-naked choke, the defence itself creates a specific body position, because there are only a limited number of ways to defend each attack safely. An experienced grappler knows what that defensive shape looks like before attempting the first submission, and treats the first attempt partly as a way of forcing the opponent into that predictable shape.
Building the chain
Once the opponent has defended in the expected way, the second attack targets the new position the defence created rather than repeating the original attempt. A defended triangle attempt, for example, commonly leaves an arm extended and exposed, which is exactly the position an armbar needs. The chain is not random improvisation; it is a sequence rehearsed in training specifically because one defence reliably produces the next opportunity.
Why chains finish fights
A single submission attempt gives an experienced opponent one clear problem to solve, and skilled defenders solve single problems often enough. A chain gives them a moving target, and the energy spent defending the first attack usually leaves less strength and composure available for the second or third. This is why commentators note that a fighter looked 'close' on several submissions in a row; the closeness is often the deliberate product of the sequence, not a coincidence.