Stimulus checks combined with broad accessibility of trading platforms (Robinhood, etc.) led to fund inflows growing 71% https://www.reuters.com/markets/europe/global-markets-etf-gr...

And that's just ETF inflows aside from mutual funds and options markets.

The top recipients, according to the article linked above, were VOO, VTI and SPY. All follow an index that's market-cap-weighed, benefiting top 10 holdings more than the rest of the market.

The explanation I've seen floated is retail investors treating VOO and VTI as their savings account (for the lack of any decent yield opportunities). That money is eventually going to be withdrawn and spent, punishing those top 10 holdings excessively compared to the rest of the market.