I saw the Panorama documentary this evening having watched the Youtube clip earlier today.
Of course, losing your temper on camera is never advisable but...
He’d just witnessed a rather sickening Scientology exhibition which hijacked the holocaust to their own nutjob philosophy and was then immediately harrassed by their oily creep of a spokesman for being a bigot. In the context of the programme it seemed like a reasonably sane response short of punching the guy out.
If you watch the whole Panorama episode in question and view the clip in context, it becomes completely unsurprising that John Sweeney blew up like he did - it was EXACTLY what the “church” had been pushing him and his entire production crew toward for the past several weeks.