Aberrant QRS morphologies during atrial tachycardia: What is the mechanism?

A 12-lead electrocardiogram (ECG) was obtained from a 73-year-old woman after laparoscopic gastric bypass surgery (Figure 1). The ECG reveals an ectopic atrial tachycardia with a cycle length of 360 ms. There is second-degree atrioventricular block in a 3:2 pattern. The second PR interval of each 3:2 period is longer than the first, so the block is of the Wenckebach (Mobitz 1) type. In every other Wenckebach cycle, the second QRS complex is conducted with right bundle branch block (RBBB) aberrancy. What is the mechanism of this pattern of aberrancy?