Medb, the queen, had previously offered Ferdiad great rewards:
And when Ferdiad was come into the camp, he was honoured and waited on, and choice, well-flavoured strong liquor was poured out for him till he became drunken and merry. Great rewards were promised him if he would make the fight and combat, namely a chariot worth four times seven bondmaids, and the apparel of two men and ten men, of cloth of every colour, and the equivalent of the Plain of Murthemne of the rich Plain of Ai, free of tribute, without duress for his son, or for his grandson, or for his great-grandson, till the end of time and existence.
Medb: "Champions will be surety,
Thou needst not keep hostings.
Reins and splendid horses
Shall be given as pledge!
Ferdiad, good, of battle,
For that thou art dauntless,
Thou shalt be my lover,
Past all, free of cain!"
Ferdiad: "Without bond I'll go not
To engage in ford-feats;
It will live till doomsday
In full strength and force.
Ne'er I'll yield-- who hears me,
Whoe'er counts upon me--
Without sun- and moon-oath,
Without sea and land!"
Medb also manipulates him by twisting his foster-brother's words:
"Ye men," spake Medb, in the wonted fashion of stirring up disunion and dissension, "true is the word Cuchulain speaks." "What word is that?" asked Ferdiad. "He said, then," replied Medb, "he would not think it too much if thou shouldst fall by his hands in the choicest feat of his skill in arms, in the land whereto he should come." "It was not just for him to speak so," quoth Ferdiad; "for it is not cowardice or lack of boldness that he hath ever seen in me. And I swear by my arms of valour, if it be true that he spoke so, I will be the first man of the men of Erin to contend with him on the morrow!"
This interprets Medb's words not just as poison, but as a threat:
Ferdiad does not want to fight Cúchulainn but he is threatened with satires about himself and his family if he does not comply to the wishes of Medb. He had already refused her gifts of a chariot and the hand of her daughter Finnabair in marriage, the threat of being a figure of fun for all eternity proves too much for Ferdiad and he reluctantly consents to fight his foster brother.
Also, Cúchulainn apparently thought there was no way out:
Thus Ferdiad went into combat albeit reluctantly. Cúchulainn greeted him in a friendly manner only to be told by Ferdiad that due to circumstances beyond his control he had come to do battle instead of renewing their friendship.
Later, he said
Cuchulain: "I'm come before warriors
Around the herd's wild Boar,
Before troops and hundreds,
To drown thee in deep
In anger, to prove thee
In hundred-fold battle,
Till on thee come havoc,
Defending thy head!"
So, in summary:
- Medb twisted Cúchulainn's words to anger Ferdiad, and alternately threatened him and promised rewards.
- Cúchulainn thought, for whatever reason, that there was no other way out, and was determined to defend those he was fighting for.