Albert Barnes 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)

Saul had come from Michmash to Gilgal, expecting to gather the force of the whole nation around him. Instead, the people fled, leaving him in the exposed plain with only 600 men (1 Samuel 13:15).

The Philistines occupied Michmash and could at any moment pour down the valley upon Gilgal. Saul’s situation was obviously one of extreme peril, and a delay of even a few hours might have proven fatal to him and his small army. Therefore, he “forced” himself to act.