How did they trap Lancelot? Why did Arthur choose that evening to go hunting?

Agravaine predicted Launcelot would stay behind as well, and then he and a select group of knights, including Mordred, could catch him in the act of adultery and detain him. Arthur agreed to the plan, but warned them to be careful. The next day, Arthur rode out to hunt, leaving word he would be gone overnight. I don't think Arthur wanted to be there when it happened.