1)
Why did they gather?
1.
Radak: When they saw that they were chasing Avner, they gathered to fight against Yo'av [to defend Avner].
2.
Malbim: They gathered for war. This is also why they stood on the hill; from there, it is easy to win.
2)
What is "Agudah"?
1.
Radak: It is a group gathered together, as if they are tied to each other.
