I think in grand tours you tend to have a strategy of "don't burn matches when you don't need to." So when an effort like end of stage 18 is strategically unnecessary since he has an insurmountable lead, someone in tune with tactics thinks, "why is he wasting energy?"
He's already turning some heads and raising some eyebrows. He could have just as easily come to the line with the other two cyclists and lost no time. He's going to win a grand tour back to back, so it's not about his brand.
Dominating a race and then unnecessarily flaunting your dominance is no way to chill doping rumors.