Any Canadians in the room should remember this as the exact mechanism by which Nortel Networks became astronomically huge. Any time Nortel got more valuable, index funds tracking the Toronto Stock Exchange (TSE) loaded up on Nortel, amplifying the price increase. This gave the company massive amounts of capital to buy other companies with, which generated more headlines, which brought in more investor capital, which brought more index funds in. In fact, at one point Nortel was so valuable it made the TSE too homogenous to legally index, at least until Nortel lobbied Canada to change the rules regarding diversified index funds.

If you aren't Canadian (like me) you can watch this Bobbybroccoli video that explains it very well: https://www.youtube.com/watch?v=I6xwMIUPHss

Spoiler alert for the Bobbybroccoli video, but it turns out this trick doesn't work forever. And when Nortel inevitably crashed it left a good chunk of Canadians as bagholders. And looking at the stock market over the past few years, where basically all the the growth is seven companies, I'm starting to wonder if we're finally seeing America's answer to the Nortel fiasco.

(No, Lucent doesn't count, even though they're literally America's counterpart to Nortel. The key factor that made Nortel a problem was the lack of diversity in the Canadian market. Lucent crashed and burned in a field of hundreds of growing big-cap stocks, Nortel was an extremely big fish in a tiny pond.)