1 Samuel 13:11

And Samuel said, what hast thou done?
This question he put to bring him to a confession of what he had done, otherwise he guessed at it by his countenance; or rather, by the prophetic spirit he was endowed with, he knew it certainly that he had offered the sacrifices without waiting for him:

and Saul said because I saw the people were scattered from me;
they were deserting, and he feared, if he stayed any longer, they would all leave him; this was one reason of doing what he did:

and that thou camest not within the days appointed;
seven days were appointed, and because the seventh day was come, though it was not gone, he concluded Samuel would not come at all; and that was another reason why he did what he did; and by this would have laid the blame on Samuel, as if he did not keep his time; whereas it was Saul's impatience that hurried him to this action:

and that the Philistines gathered themselves together to Michmash;
where his station before was, and from thence he might quickly expect them at Gilgal; and this was another reason why he hastened the sacrifice.