1)
Why did they need to say that Hashem abandoned him?
1.
Rashi (10): Therefore they will not be punished for [harming] me, for I already stumbled in sin.
2.
Radak: Until now, Hashem helped him, and he succeeded in his wars. Now it seems that He abandoned him - even his son pursues him to kill him, and no one saves him!
3.
Malbim: Therefore "chase, and You will catch him."
2)
To whom do they say "chase..."?
1.
Radak: Each of them says to his colleague, like it said "No'atzu Yachdav" (verse 10).