1 Samuel 10:20

And when Samuel had caused all the tribes of Israel to come near, the tribe of Benjamin was taken.

At Mizpeh the assembled people prayed for guidance, and then the tribes were brought forward in a solemn lot to reveal publicly whom God had chosen as king. The choice was not made known by immediate announcement but proceeded stepwise, with all the tribes brought near and Benjamin taken, then narrowed through family and household until Saul was identified as the chosen man.

Benjamin was the smallest tribe, so taking the king from it slighted Judah and Ephraim as the largest and most powerful tribes. That slight stirred opposition to Saul among those who had favored other candidates or wanted the throne for themselves.