It’s their job. That’s what they’re paid to do. They have experience in what triggers cause what responses.
It’s not usually a terribly mysterious process. Most of the trading is done by a relatively few enormous investors. Everybody knows everybody else, everybody talks to everybody else. They use computer models that are very similar to one another’s. Certain reports, of statistical indicators, of values, of quarterly earnings, of federal actions, occur at known times and whether they are up or down will govern actions in the same direction virtually every time.
It’s like asking lawyers how a Supreme Court decision will affect the outcome of future cases. Most will give similar answers because that’s been the pattern in the past. And they know the patterns of the past because they spend their whole professional lives analyzing them.
It’s not a case in which everybody counts equally. Those millions of investors mostly don’t exist - they’re invested in 401Ks and pension plans and mutual funds in which a few people do the investing for them. The small investors who try to do it for themselves can be discounted because they’re a fraction of a percent of the whole. Getting a read on what those few who count do is not rocket science.
Whether what they do is correct or not is a different and far more difficult thing to figure out without hindsight. But figuring out why the herd breaks right or left is something any cowboy can discover.