Israel celebrates rescue, most probably from the Babylonian exile.
Though Israel used to be oppressed by their enemies, they were unable to gain victory over Israel.
Psalm 129:4 But the LORD is righteous;May all those who hate Israel be put to shame, and may they be short-lived. May they be like grass growing on rooftops, which a reaper cannot fill his hands with.
He has cut me free from the cords of the wicked.
Psalm 129:8 May those who pass by not say to them,
“The blessing of the LORD be on you;
we bless you in the name of the LORD.”
Only a righteous man will thank LORD for cutting his cords free from the wicked. All others will enjoy their company and are on constant watch towards that next opportunity to be in their presence.
The enemies treated Israelites like dirt and walked over them, and as the author puts it, plowed their backs. When others treat us like dirt, we gotta follow the ABC rule:
Benefit from it,
Commit to the LORD.
The blessings mentioned towards the end of the psalm is aimed at grass growing on rooftops. Nobody in their right mind will use such language in reference to grass growing on the roof. It will be absurd to use such blessings upon such worthless and withered grass.