Setting aside the geographic references to a rich and pleasant land, "The bulls of Bashan" has two potential scriptural references: Amos 4, and Psalm 22.
In Amos 4 it is a dig at the women in the northern kingdom who lived lavishly while oppressing the poor and boasting in their holiness. Their judgment was coming, and it did with the Assyrians conquering them. As it says:
Hear this word, you cows of Bashan on Mount Samaria,
you women who oppress the poor and crush the needy
and say to your husbands, “Bring us some drinks!”
The Sovereign Lord has sworn by his holiness:
“The time will surely come
when you will be taken away with hooks,
the last of you with fishhooks.
You will each go straight out
through breaches in the wall,
and you will be cast out toward Harmon,”
declares the Lord.
“Go to Bethel and sin;
go to Gilgal and sin yet more.
Bring your sacrifices every morning,
your tithes every three years.
Burn leavened bread as a thank offering
and brag about your freewill offerings—
boast about them, you Israelites,
for this is what you love to do,”
declares the Sovereign Lord.
In Psalm 22 it appears to be deep within a context. If we accept that Psalm 22 is prophetic and is referring to Christ on the cross, then we have enough information to reach a pretty solid conclusion. The reference to the "Bulls of Bashan" appears to be Jewish council that Christ appeared before at his trial. They attack his reputation, and roar against him.
Many bulls surround me;
strong bulls of Bashan encircle me.
Roaring lions that tear their prey
open their mouths wide against me.
The previous section is to be contrasted to the next group of people, the dogs (unclean animals). These are the acts against the speakers body. They pierce the hands and feet, and divide the clothes and cast lots as the people look on. These are the Romans soldiers.
Dogs surround me,
a pack of villains encircles me;
they pierce[e] my hands and my feet.
All my bones are on display;
people stare and gloat over me.
They divide my clothes among them
and cast lots for my garment.
The common theme being "Bulls of Bashan" is a reference to people living lavish lifestyles and while putting their false holiness on display. Both groups of people were cast out of the land violently.