John Gill Commentary 1 Samuel 13:11

"And Samuel said, What hast thou done? And Saul said, Because I saw that the people were scattered from me, and that thou camest not within the days appointed, and that the Philistines assembled themselves together at Michmash;" — 1 Samuel 13:11 (ASV)

And Samuel said, what have you 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 you came 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.