A short RP tachycardia in congenitally corrected transposition. What is the mechanism?

A 51-year-old female with congenitally corrected transposition of great arteries (CCTGA), situs solitus, dextrocardia, atrial septal defect and persistent left superior vena cava underwent electrophysiology study for recurrent palpitations with documented narrow complex, short RP tachycardia. With a catheter in the region of the anterior mitral annulus, a His signal was recorded and HV interval was 35 msec. Tachycardia was induced with a ventricular extrastimulus. During the tachycardia there was 1:1 ventriculo-atrial conduction and central atrial activation with a VA interval of 20 msec. The recorded His signal could be seen after the QRS. What is the mechanism of the tachycardia?