Essentially their separation was caused by three factors which are recorded in verses 5-7 of Chapter 13…
Now Lot, who went with Abram, also had flocks and herds and tents and the land could not sustain them while dwelling together; for their possessions were so great that they were not able to remain together. And there was strife between the herdsmen of Abram’s livestock and the herdsmen of Lot’s livestock. Now the Canaanite and the Perizzite were dwelling then in the land (Genesis 13:5-7).
The first problem was the success of both men as keepers of flocks. Both Abram (13:2) and Lot (13:5) had prospered. Now their flocks and herds had become so large that they could no longer dwell together (13:6). This was especially true for nomadic tribesmen who must travel about looking continually for pasture for their sheep and cattle.
The second problem was the strife which seemed to be steadily growing between the herdsmen of Abram and Lot (13:7). Each man’s herdsmen sought water and the best pasture for the animals of their master. This competition inevitably led to conflict between the herdsmen of Lot and Abram.
It would probably not be far from the facts to suggest that some irritation already had become evident between Abram and Lot themselves. This may be implied by Abram’s words in verse 8. This also would be true to life. Whenever there is contention between followers, there most often will be strife between the leaders also.
If the first problem is the success of both Abram and Lot, and the second is the resulting strife, the third is the fact that the land where they sojourned was shared with others; namely the Canaanites and the Perrizites (13:7).