Why could he just not stand silently?
One must consider the symbolism of raising one's fist, ala John Carlos and Tommy Smithin Mexico Olympics in 1968, as mentioned in the article.
The raising of the fist was then and is now, a symbolic gesture of Black Power, declaring the USA to be a racist state. Although no mention of race was made (that I saw) my guess was that these were Black Students, thus making the gesture very symbolic and very offensive.
What if they had raised their hand in the classic one finger salute? Would you consider that to be disruptive?
If the answer is yes,then the same applies for a clenched fist.