After Samuel anointed Saul, he sent him home with a few signs that will be fulfilled in 1st Samuel 10:9-13 (NJPS):
As [Saul] turned around to leave Samuel, God gave him another heart; and all those signs were fulfilled that same day. And when they came there, to the Hill, he saw a band of prophets coming toward him. Thereupon the spirit of God gripped him, and he spoke in ecstasy among them. When all who knew him previously saw him speaking in ecstasy together with the prophets, the people said to one another, “What’s happened to the son of Kish? Is Saul too among the prophets?” But another person there spoke up and said, “And who are their fathers?” Thus the proverb arose: “Is Saul too among the prophets?” And when he stopped speaking in ecstasy, he entered the shrine.
It sounds like the people didn't think Saul was the sort of man would would be caught up in prophetic ecstasy, which is why they asked if he were among the prophets. He was acting out of character. But why do they ask "And who are their fathers?" about the other prophets? Were they assuming Saul hired these fellows to "make a good show"? What's going on here?
And what is the meaning of the proverb: "Is Saul too among the prophets?" Is it along the same lines as Jeremiah 13:23 (NJPS):
Can the Ethiopian change his skin,
Or the leopard his spots?
Just as much can you do good,
Who are practiced in doing evil!