In the generality, I think intergroup selection of paternal lineages is the answer to why star-shaped phylogenies are so evident in the phylogenetic record ~4,000 years ago. More precisely, most of the major clades of R1a, R1b, and I1 undergo massive expansion after a sharp reduction in effective population size around this period. The R lineages diversified during the Pleistocene, probably in Central Eurasia (it is a brother clade to Q). The I lineage derives from Western European hunter-gatherers, probably the late Pleistocene expansion which eventually gave rise to the Mesolithic groups that encountered the early farmers.
But what happened here specifically? Let me quote a section of Peter Turchin’s excellet Ultrasociety: How 10,000 Years of War Made Humans the Greatest Cooperators on Earth:
Lanchester’s Square Law yields an enormous return to social scale. If the opposing forces use a mix of ranged and shock weapons, numerical superiority will still be amplified, although not as much as with purely projectile weapons. So there is an intense selection pressure for cultural groups living in flat terrain to scale up, and a very high price to pay by those that fail to do s….
Though human interaction with horses as domesticates is probably older, light chariots emerged on the Pontic steppe ~4,000 years ago. Within a few centuries, this technology was ubiquitous in the Near East. The Indo-Aryan Mitanni arrive with chariots in modern Syria/Northern Iraq by ~3,750 years ago.
In the Near East chariots and bows were closely associated. The evidence from the Eurasian steppe during the Bronze Age seems less definitive (simply, bows may not preserve very well), though by the Iron Age the mounted archer became a ubiquitous feature of the military landscape.
The combination of chariots, likely bows, and the Sintashta/Srubna/Andronovo culture’s known focus on metallurgy, make it hard for me to deny the likelihood that the expansion of R1a1a-Z93 has something to do with intergroup conflict. The reality is that Lanchester’s Square Law means that even small initial advantageousness for a given paternal lineage will probably snowball. One victory will lead to an increase in territory and resources, which will produce later advantage. A sort of Y chromosomal Matthew Effect.
But this doesn’t explain what occurred in Europe, where R1b and I1 also underwent a massive expansion (and R1a as well). Europe’s relatively forested territory beyond the Hungarian plain always blunted the power and reach of mounted archers later in history. We do know that chariots arrived in the Mediterranean around the same time as in the Near East. But the rise to dominance of the Corded Ware and Bell Beaker peoples predates light chariots. Perhaps it is something as simple as the fact that metaethnic institutions and identities that could dampen intergroup conflict hadn’t emerged, but it’s still curious to me that one could have a ~90% population replacement in Britain in a few centuries.
Perhaps we will find out that it has to do with a disease as our understanding of ancient epidemics gets better.