Dunk had only wanted to participate in the tourney at the Ashford Meadow to honor Ser Arlan of Pennytree, he had not desired a squire or the mess he found himself in. But Dunk was nothing if not a man of his oath, and as a knight that oath was "protecting the weak and defending the innocent."
Which was exactly what you, a knight there to compete in the tourney as well, saw the evening that Dunk was arrested. He had saved the pretty puppeteer Tanselle from the all but deranged Prince Aerion.
This of course led to not just Prince Aerion, but his elder brother Prince Daeron's accusations.
According to the two Princes, Dunk as not only committed treason against Aerion by preventing him from dealing corporal punishment, but the hedge knight was accused of kidnapping his new young squire Egg, whom he had no clue was the youngest of Prince Maekar's sons, Aegon.
Instead of just imprisonment or death alone, Prince Aerion had called for a trial by seven, in which the offending persons, Dunk and Aerion collected six companions to engage in combat together until death or surrender.
That is how Dunk officially was greeted by you, recruited by the young Egg to help in Dunk's fight along side five others, including Prince Baelor, Egg's uncle.
After the fight had ensued, few were dead, Aerion had surrendered to Dunk, and Prince Baelor, unfortunately was mortally wounded.
Brushing yourself off, recovering from rather poor blows from Prince Daeron, and another of Aerion's men, you shuffled back to the campgrounds of the tourney, finding Duncan the Tall nursing his own wounds under an elm tree not to far from the main camp.
As you approached, Dunk glanced up, having expected Egg or even Tanselle. But he had not been expecting you...the unnamed knight who had aided him in his trial.
"I owe you thanks, Ser," The large man said, his voice deep as he glanced between the wrapping he twisted around his arm, and you standing above him, "Your name?"