Interesting case of narrow QRS tachycardia. What is the mechanism?

A 67-year-old female presented with history of recurrent palpitations. During one of the episodes of palpitation, a narrow QRS tachycardia was recorded and it was reported to be terminated with intravenous adenosine. The 12-lead electrocardiogram (ECG) showed no manifest ventricular preexcitation. ECG was within normal limits. The patient underwent an electrophysiology study after informed consent. She was taking calcium channel blockers and that was stopped five half-lives prior to the procedure. As catheters were being placed, a narrow QRS tachycardia got induced. During the study, there was development of right bundle branch block during the tachycardia. What is the mechanism of the tachycardia?