The rest of the market is leaving Big Tech behind after a historic comeback

September's rally was concentrated in artificial intelligence names, and that narrow leadership flattered the headline index while much of the market lagged. October has inverted that picture: the AI trade that carried the market is now the drag, and the rest of the market is outperforming the former leaders. That rotation, rather than a broad collapse, is the defining feature of the month.

The distinction matters for how investors read index-level moves. When gains rest on a handful of mega-cap technology companies, the index can look healthy while breadth quietly deteriorates. A reversal in those leaders can therefore hit the benchmark hard even if most individual stocks are holding up. October is testing that dynamic in real time.

What to watch is whether the broadening persists. Sustained participation from sectors outside technology would suggest a healthier foundation for the market. A brief rotation that quickly snaps back to the prior leaders would suggest the concentration risk never really left. Breadth, not just the headline number, is the signal worth tracking.