The Athenians had partially destroyed the smaller town of Oropos, which was under their nominal control. The writer Pausanias, much later, thought it was because the Athenians were experiencing economic hardship after the Macedonian wars. The people of Oropos appealed to Rome, as the de facto hegemon, for help, and the Roman senate decided to let the city of Sicyon be the impartial judge. The Athenians did not bother to show up for the trial, and so Sicyon fined them 500 talents (which they never paid). Pausanias sums it up:

While [Gallus] was carrying out his instructions, the Athenian populace sacked Oropus, a state subject to them. The act was one of necessity rather than of free will, as the Athenians at the time suffered the direst poverty, because the Macedonian war had crushed them more than any other Greeks. So the Oropians appealed to the Roman senate. They resolved that an injustice had been committed, and instructed the Sicyonians to inflict a fine on the Athenians commensurate with the unprovoked harm done by them to Oropus. When the Athenians did not appear in time for the trial, the Sicyonians inflicted on them a fine of five hundred talents, which the Roman senate on the appeal of the Athenians remitted with the exception of one hundred talents. Not even this reduced fine did the Athenians pay, but by promises and bribes they beguiled the Oropians into an agreement that an Athenian garrison should enter Oropus, and that the Athenians should take hostages from the Oropians. If in the future the Oropians should have any complaint to make against the Athenians, then the Athenians were to withdraw their garrison from Oropus and give the hostages back again. [Paus 7.11.4-5]

Athens sent the three philosophers you mention to Rome as envoys to appeal their side of it. The main source is Plutarch, in the context of Cato the Elder, who, being his normal grumpy self, was not well pleased to have these Greeks putting on spectacles of academia in his city. The Roman youth are said to have gone mad for philosophy, and Carneades in particular made quite an impression. The dude even suggested that Rome, if they were to act morally, should return all their imperial conquests and return to their ancient lives as shepherds living in huts in the countryside of Latium.

See Plutarchy, Cato the Elder, chapter 22 and following. Cf Cicero de Oratore 2.155, Pliny the Elder NH 7.112-113