In order to study this I combed through last year's discipline stats. While Arabe Unido had the most red cards of any team in the 2009-2010 edition of the tournament, they weren't even in the top five as far as yellow cards per game. After factoring out the teams that only played twice, these teams rose to the top:
YC/G RC/GSo three Mexican teams, three American teams and then Arabe Unido. Perhaps there's something about playing American teams that brings out the worst in Arabe.
Pachuca 3.70 0.00
Houston 3.17 0.17
Toluca 3.00 0.00
Cruz Azul 2.90 0.10
DC United 2.63 0.25
Columbus 2.38 0.13
Arabe Unido 2.30 0.30