1)
Did Galyus approach David only now?
1.
Radak: When he heard David's words, his anger flared and made himself zealous to approach him. He could not go quickly due to his armor.
2)
Why did David run?
1.
Malbim: He ran back, to the side of the battlefield, and positioned himself to face him, in order to shoot the slingshot at the target.
