Why would someone get a capsular contracture 2-3 years post augmentation with normal mri's?
Everyone who gets an implant forms a capsule around the implant. It's just scar tissue, and it's the body's way of "walling off" the implant. The vast majority of the capsules that form are Grade I, which means it's soft and you don't even know it's there. Grade II capsules are firm, but otherwise OK. Grade III capsules are firm but also distort the breast, and Grade IV capsules are firm, distorted, and painful. Grades III and IV are considered "capsular contractures" by virtue of the fact that they distort the breast.
The MRI's only show the integrity of the implant - meaning, if it's ruptured or not. And while you can definitely get a capsular contracture from a rupture (as the body is trying to wall off the ruptured material), that is not the only cause. Low levels of bacterial contamination can cause an inflammatory response that is not enough to cause a full blown infection per se, but enough to cause a reactive response which results in a contracture. Similarly, low levels of blood or fluid - either immediately postoperatively, or after a tear in the capsule years later by straining - can lead to a capsular contracture. Muscle strain from overexertion can cause a contracture as well. Often times, we don't know the reason for why a capsular contracture results. These are called "idiopathic", and may occur for the same reason that some people just scar worse than others - they just have a predisposition to it happening.