A 61-year-old man presented to the Emergency Department after receiving multiple shocks from his cardiac resynchronisation therapy-defibrillator (CRT-D) device, implanted 2 months previously for New York Heart Association (NYHA) class III heart failure and left bundle branch block (LBBB). He had hiccoughs and was anxious, but denied chest pain and dyspnoea. He had chronic atrialfibrillation (AF). He was haemodynamically stable but received several further unheralded shocks. The cardiac monitor appeared to show AF and LBBB throughout. His presenting chest radiograph is demonstrated in figure 1.

Fig 1. The ED chest radiograph.

Which of the following is the most likely cause for this man's CRT-D firing? 

(Please select 1 option)