It is important to distinguish atrial hypertrophy and atrial enlargement.  Atrial hypertrophy (i.e. thickening of the atrial wall) is rare, as there is very little muscle in the atrial wall.  Perceived hypertrophy may actually be increased wall thickness due infiltration or deposition where non-muscle products develop in the atrial walls.  It is incredibly rare to encounter a problem where atrial walls are thickening enough to be clinically important.

On the other hand, atrial enlargement (ie. increasing diameter of the atrium) is very common and is often found in aging hearts.  Associations with atrial enlargement include late-stage mitral valve diseases such as stenosis (decreased opening size) or regurgitation ("leaky valve").