In general, look up what constituted a valid reason for dueling in the Middle Ages. For example, here:

To illustrate the judicial duel I will give a synopsis of the account of the combat at Moulins, France on February 17, 1538. During this duel Lion de Barbencois (Sieur de Sarzay) did combat François de Saint-Julien (Sieur de Veniers). A quarrel had been in progress for many years between Sarzay and a gentleman by the name of Sieur de La Tour-Landry. Sarzay had sworn that La Tour-Landry had fled like a coward during the battle of Pravia in 1525 (where Sarzay was not even present). La Tour-Landry's honor was placed in jeopardy by this and demanded that Sarzay give the source of this information. Sarzay related that he had gotten the information from Veniers who vehemently denied the accusation of supplying the information; thus giving Sarzay the Lie. Now, the quarrel changed from Sarzay and La Tour-Landry to Sarzay and Veniers. Thus the duel was not held to clear La Tour-Landry of the charge of cowardice on the field but to dispel the suspicion of a lie. Gossip true, but a lie. In this duel, Sarzay was the injured party since he was accused of lying. After the challenge was given the King was petitioned for a field.

The Wikipedia page on Duel also gives a vivid flavor of the time.