Unless they are massively gaining market share and sales, thereby increasing their intrinsic value.
More likely, it represents a flight to quality. There is actually nothing wrong with that, if you already own, and if the costs of ownership don't rise insanely.
You see this in many other markets as well. Take the real estate market, the top properties in the most desirable areas go up the most, while other market segments stagnate or actually fall, with the worst-of-the-worst losing even any speculative value they could have had. The worst-of-the-worst are picked up in liquidation for either disposal or turnaround/absorption, just as companies are. I'm not seeing a lot of de-listing going on, but in certain industry sectors there is still a lot of buying of smaller entities, which is one way the big guys grow--those properties are actually worth more in their hands than they were in the hands of the previous owners.
The higher you go in quality, the less real value you get. Consider RE again. At the top, nobody buys as an investment, they buy because they can, don't care about selling at a significant loss, and are positioned to absorb that loss. The top tends to be overpriced because it is as much about prestige, neighborhood, and security as it is about returns.
Obviously prices will rise when everybody wants in, which is what might be happening with blue chips. Bonds suck. Small-caps suck. Emergings suck. Europeans suck.
Heck, just talking about it is making me want to move into exclusively the blue-chip neighborhood when I get back in.
Of course there are many other dynamics at work, but I do think that this one is having an effect.
Down 42 today to 17,071, nothing really significant.