Looking at the books, Geralt is frequently challenged simply because he got a reputation as a great swordsman, and because he is famous. Whoever beats him in a duel will become famous in turn.
At one point there's a another reputed fighter, Cicada, who keeps insisting on a duel with Geralt. Who finally had enough when Cicada caught him in a foul mood and beat up Cicada in humiliating ways. Similarly, another witcher called Brehen also challenges Geralt because of his fame, but Geralt refuses the challenge.
As for the game's character Ronvid, he is very likely inspired, or possibly even the same character, as an unnamed knight in the books who was hoping to pick a fight with Geralt. From The Lady of the Lake:
A colorful tent ressembling a cake stood outside the city, not far from the turnpike. A white shield with red chevron hung on a pole in front of it. A knight in full armour and a white surcoat decorated with the same arms as the shield was standing under the raised flap of the tent. /--/
His eyes lit up in hope at the sight of Geralt and Dandelion riding slowly along.
"The lady of your heart-" Geralt dispelled the knight's hope in an icy voice "-whoever she is, is the most beautiful and most virtuous virgin from the Yaruga to the Buina."
"By my troth," the knight snapped back. "You speak the truth, sir."