Is it that damned crooked finger with which Sebastian Vettel always signifies yet another victory or pole position? Is it team boss Christian Horner's banal and monotonous message: "Fantastic, Seb, you've just won the Whatever Grand Prix"? Or is it perhaps what Vettel did so ruthlessly to team-mate Mark Webber in Malaysia?
Everyone has their own theory as to why the triple world champion has been getting booed after races. It happened in Melbourne (OK, Webber country); when his gearbox failed during the British GP at Silverstone; again in Belgium (possibly more against Greenpeace's protest); then in Italy and Singapore. And it may happen today at the Korea International Circuit.
Part of the answer is obvious. People like variety. Vettel has won 33 grands prix, including seven of this season's 13 races, which means he will be world champion for a fourth season running.
Adrian Newey leads the team that designs his cars, and Newey is a genius. It is no longer possible to be innovative in terms of major design breakthroughs, but while the things Newey brings to the Red Bulls might of themselves not be spectacular, they are brilliantly exe-cuted – and brilliantly effective.
"Everyone knows the Red Bull is good aerodynamically," Lewis Hamilton said after beating Vettel in Texas last year. "But it also has fantastic mechanical grip." That was never more apparent than in Singapore recently, where at times Vettel was able to pull away by as much as two seconds a lap.
Early this year Red Bull gave their engine supplier, Renault, detailed specifications for each of the five electronic set-ups (known as maps) that teams are allowed to run. Newey had come up with the things he wanted to do with the exhaust system in order to continue enhancing rear-end downforce, but he also wanted a means of limiting the engine's torque in the initial opening of the throttle.
Renault obliged. At first the optimal map was too aggressive for Pirelli's softer-compound rear tyres. But since the specification of their rubber was changed after the British GP, the harder-compound tyres have really suited that map.
It worked in Belgium and Italy, and really worked in Singapore, which has more corners than any other circuit. As others struggled with wheel-spin, Vettel's car stayed planted and off he went.
"I followed him for a little bit on Friday morning," rookie driver James Calado said here yesterday, "and I also followed the Mercedes cars. The Mercs were sliding around but the Red Bull was just on rails."
It doesn't help Vettel that he is thus perceived to have by far the best car, as if somehow that makes it unfair. But it is clear for those who appreciate such things that he is doing a superb job of squeezing everything out of his machine without making any errors.
However, part of the problem is that Red Bull, once the paddock mavericks, have become the Establishment and are therefore fair game. Many other teams believe that Red Bull are outspending them by a significant margin, and are not operating within the spirit of an agreement restricting technical resources.
Some feel that Red Bull's hugely impressive success has bred an arrogance that is unpopular, similar to McLaren in the Lauda/Prost and Senna/Prost eras, or Ferrari at the height of Michael Schumacher's triumphs.
Yesterday Vettel was waggling that finger again after taking pole position for today's Korean Grand Prix, but it was a close-run thing, as Hamilton in his sliding Merc was only 0.218sec adrift.
But if what Vettel perceives as a "travelling band" of booers is here, they will likely be doing their thing again this afternoon as the German crosses the line first once more.