1)
Above (3:12, 4:1) it said that Yisrael added to do evil. Why does it not say so here?
1.
Rashi (from Midrash Tehilim 18:6, Shir ha'Shirim Rabah 4:3): Before, sins were added to sins. Through this Shirah, all their sins were forgiven, and now they began to sin.
2.
Malbim: In the days of Devorah, they repented totally. Afterwards they began to sin afresh.
