1)
Who stumbled and perished?
1.
Radak: All the ranks of Pelishtim fled and turned back - "va'Yir'u ha'Pelishtim Ki Mes Giboram va'Yanusu" (Shmuel I, 17:51).
2)
Why does it say "Mipanecha"?
1.
Malbim: After my enemy fled, this that they stumbled and perished a second time, was not chance - it was due to You.
