Meliagrance knows Lancelot will so be after him, so he sets up an ambush on the road leading to his castle.
He's also trying to swank up his castle to be worthy of Guenever (he genuinely loves her, and isn't really a bad guy—he's just made up a plan that's now gone too far for him to back out of).
Lancelot, having been ambushed, rides the rest of the way to Meliagrance's place in a cart—which is totally disgraceful back then, since only prisoners and criminals ride in carts.
When he sees Lancelot arrive, Meliagrance begs Guenever to save him from the knight. He yields!
She agrees, and Meliagrance asks her to stay at his castle for the night, and they can get everything straightened out in the morning. Besides, she could just say she was there for a friendly visit, and no harm no foul to her reputation.
Maybe it's the change of scenery, but when Guenever goes downstairs and sees Lancelot, the sparks fly once again. She's happy to see him alive, even if it means he will choose God over her. She just wants Lancelot to be Lancelot once again.
Guen tells him that she and Meliagrance have made up, so what's the fuss all about? Anyway, fight him if you want to. I don't care.