The Hebrew Bible in (Genesis 13:7) says only that there was strife between the shepherds. Targum Pseudo-Jonathan tells you what the strife was about, and the answer is an ethics lesson disguised as a range dispute.
The Aramaic reports a sharp divergence in livestock practice. Abram has taught his shepherds a rule: do not let the cattle go among the Kenaanaee and the Pherizaee, who still have power in the land, and do not let them make depredations on the pasture of others. Abram's men muzzle the animals. They do not graze where the grass belongs to someone else.
Lot's shepherds reason differently. The land has been promised to Abram's seed, they say, and since Lot is Abram's heir (so they assume), the pasture is practically theirs already. They let the cattle roam. They graze on land that is not yet theirs.
This is the real strife. It is not a fight over water. It is a fight over whether a promise for tomorrow justifies theft today. Abram's ethic is strict: until the land is given, it is not taken. Lot's camp argues that the covenant is a license.
The Targumist is drawing a line that will run through the entire Hebrew Bible. Promised futures do not entitle us to present sins. A covenant is not a coupon. The land belongs to its present inhabitants until the Holy One works the transfer Himself. Abram, the patriarch of patience, enforces this in the way shepherds handle their livestock — a daily, mundane discipline that turns out to be the architecture of a people.
The separation that follows in (Genesis 13:9) is not estrangement. It is the recognition that two men with different ethics cannot share one caravan.