Complex sentence: They also waved tiny American flags, infuriating fans, who questioned their sincerity, and humorously attempted to start "USA!" chants, which led to further negative fan "heat".
Split into simpler sentences: They also waved tiny American flags, infuriating fans, who questioned their sincerity. Also, they would humorously attempt to start "USA!" chants, which led to further negative fan "heat".