According to history, Cato had long borne a grudge against Caesar for publicly embarrassing him. In brief, when they were both in the Senate, Cato spotted Caesar reading some correspondence while Cato was speaking, which was a pointed insult. Cato called on Caesar to read the letter aloud, which Caesar at first refused to do. Cato then invoked a public demand to hear the letter, whereupon Caesar read aloud an erotic love letter, which had been written by Cato's own half-sister, Servilia (one of Caesar's many lovers at the time). Cato never forgave Caesar for the insult.
Caesar's need for the stolen eagles' return was not simple vanity. Like the samurai's sword, the eagle was regarded as the soul of the soldiers. By Ancient Roman law, if the eagle was lost, the entire legion was disgraced and should be broken up.
Cato and Pompey were not only close friends, but former comrades-in-arms. Caesar had once served under Pompey's command.
The word "trivia" has its roots in ancient Rome. When couriers took messages from Rome to other destinations, they were also to post brief news items about happenings in Rome at every three road intersection. In Latin that would be "tri via." Now it means basically useless but interesting information. (Guess it really always meant that.)
Posca and the slave trader are arguing about the pricing of the Gauls in the slave market, which is unsurprising. Slaves were the most lucrative part of the loot after a province was conquered.