Why does it say that David left the flock with a guardian?
Malbim: This answers Eli'av's accusation (verse 28) "with whom did you leave the flock"?
Why does it say that David went like Yishai commanded him?
Malbim: This answers Eli'av's accusation (verse 28) "why did you descend to here"?
What is "ha'Magalah"?
Rashi: They make a circle around the camp, and one who leaves is liable. If he leaves on the side of the enemy, the enemy might kill him! If he leaves on the other side, he seeks to flee!
Radak: It is a barricade, like Yonason translates 'Karkoma' (this is not in our text), like Targum Onkelos of "u'Vanisa Matzor" (Devarim 20:20). They build a barricade around the camp of planks and chains lest someone leave without permission of the officer. Chazal 1 say that a Karkum has bells, chains and dogs outside, to frighten the city or the other camp.
Yerushalmi Gitin 3:4: There are bells, chains, dogs, geese and chickens. Korban ha'Edah - this is so noise will 'announce' when one goes there.
When did the soldiers go to the battlefield?
Malbim: Via Hashgachah, this was just when David arrived, so his brothers were not on the barricade and David needed to run to the battlefield , and he saw Galyus.
