Rashi: They were tried from exertion of the journey. They were zealous to fulfill David's command, and rested only after they spoke to him.

Rashi based on Targum Yonasan: [After speaking] they ceased to speak until they heard an answer.

Malbim: This teaches his lowliness. Even though the messengers said all the words of appeasement and supplication, in the name of David, and did not add anything to anger him, he disgraced them.