What is the meaning of "va'Yaham... l'Fi Cherev Lifnei Barak"?
Mahari Kara: Hashem made them hear thunder from Shamayim (Malbim - Sisera heard chariots and horses) and terrified them, and they shoved and stabbed each other before Barak came.
Radak (17): Hashem confused them, and enabled Barak's army to defeat them.
Why did Sisera descend from his chariot to flee? Surely his chariot went faster than he can run!
Ba'al ha'Turim: [The horses could not run properly because] their hooves came off (refer to 5:22:1:1).It says here va'Yaham, just like at Keri'as Yam Suf.
Perhaps he did foolishly due to Hashgachah, or amidst panic (PF). Also Aram panicked and left their horses and fled by foot 1 ! (Ralbag Melachim 2:7:7)
Perhaps Sisera thought that those chasing him 2 can go faster than his heavy iron chariot, so he thought that his only hope is to flee covertly by foot (refer to 4:17:1:1. - PF)
Radak (Melachim 2:7:7) implies that they feared to take the time to mount their horses (Aram were not on their horses when they panicked. Surely, it would have been wiser to spend a minute to mount a horse to be able to flee faster, unless you will say that men run better than horses in the dark; it was the start of the night. - PF)
