Rashi: They are the heads of the thorns of the field. When winter comes, they are uprooted and ruined by themselves, and they fly a little [in the wind]. The verse compares what is detached from them to wagon wheels; the wind takes them.

Radak: It is straw that rolls due to the wind.

Malbim: It is a wheel. A wheel rolls by itself (downhill - PF), and wind scatters straw. So [among the enemy] a man's sword was on his colleague, and they were also destroyed due to Yehoshafat, who chased them.