Good one Eric! I've noticed the same thing too and always thought it sounded ridiculous. In fact, just before I visited the forum right now and saw your thread, I saw this on the Money/Finance section on MSN: "Stocks Rally After 3M, Caterpillar Profits Beat Estimates; Dow Up 250" (it's probably still there right now since it was there just 10 mins. ago).
Really, two companies had decent earnings and it caused the DJIA and the entire S&P 500 to go up 1.7%???
Shockingly though, this is decently accurate. The dow only has 30 or so companies, and 3M and Caterpillar are two of them. If both of them report excellent blah blah, they have a fairly large effect on the dow. That in turn makes a bunch of morons / algorithms exploiting morons to say "Shit going up? up? up! UP! BUY! BUY!" and then the dow goes up even more. And the dow is a decent proxy for the rest of the market - it mirrors it, to a large extent, so it's hardly surprising to see the rest go up too.
But wait, it's not morons all the way up. People will look at 3M and Cat to see why they're doing well. Is it because there's large demand for their products? Solid hiring, not too much turnover impacting labor costs? Is it that they're getting higher prices but paying less for materials? I don't know. But any of those and a million other reasons besides might be "evidence that similar companies should do well" or "evidence the economy is doing well" or whatever. Large demand for cat products means lots of heavy industry is gearing up, which means construction for business and homes, which means more home sales, which means more purchases of home building materials, and it means more businesses are being built / expanded, so they're doing well, and it means more people are buying homes, so they're doing well, and yada yada yada yada yada ...
Shit is super complicated and people look for pointers, hints, and proxies. How well new homes are being built and sold is a decent proxy for a whole slew of things. Naturally, how well heavy machinery that is used for all sorts of construction, mining, and other industry sells is a decent proxy for a whole slew of things.
Which is of course "journalists" get away with such bullshit headlines sometimes. Because sometimes they sound bullshit but are actually decent reasons.
Another example: some shit happens (shootings, ebola) and now uncertainty and fear goes up. That means market swings, generally, in some direction. Stocks down? Ebola. Stocks up? Ebola vaccines. Fuck it, who knows.