What is one possible reason why the Scarlet pimpernel yells “ a bad les aristos!” louder than anyone else?

The Scarlet Pimpernel joins in with the mob yelling, "À bas les aristos!" in order to blend in, to keep from attracting attention.

They, themselves, of course, never suspected who their driver was. He drove them right through a line of soldiery and a yelling mob, who were screaming, ‘À bas les aristos!’ But the market cart got through along with some others, and the Scarlet Pimpernel, in shawl, petticoat and hood, yelled ‘À bas les aristos!’ louder than anybody. Faith!” added the young man, as his eyes glowed with enthusiasm for the beloved leader, “that man’s a marvel! His cheek is preposterous, I vow!—and that’s what carries him through.”