The Victory of Yahweh (Judges 5:18-22).
“Zebulun were a people who jeopardised their lives unto death,
And Naphtali on the high places of the field.”
These (including Issachar with Naphtali) formed the main bulk of the units which climbed Mount Tabor, ‘the high places of the earth', from which they could swoop down on their foes. They were the ones prepared to take the main bulk of the fighting.
“The high places of the field” may have been poetic licence as the singer looked at the hills among which was Tabor. Or it may even be that the battle did take them into the hills, seeking the fleeing enemy. Alternately it may poetically signify the hottest part of the fighting.