With MLPs the investing flavor of the month (and of the year, and of the decade), it’s time to get as  comfortable as we can with a phenomenon that prolongs every bull market, right until it kills it.

The culprit is reflexivity. At its most basic, this is the property exhibited by every feedback loop and pair of mutually reinforcing tendencies. Reflexivity is the centerpiece of a general theory long advanced by George Soros, who’s been particularly interested in its tendency to reinforce market trends.

Typically, the opinion that a stock, industry or asset class is cheap leads to buying, which in turn generates a perception that the trend is up, which in turn generates additional buying, which can in turn alter the fundamentals that sparked the buying in the first place.

Reflexivity is no scarlet letter: it is a key ingredient of every durable bull market. But it’s also a sentence, because a trend continuously reinforced by participants’ expectations about the trend inevitably leads to actions that can no longer be objectively justified, at which point it’s only a matter of time until a different trend asserts itself.

Does any of this matter for MLPs and MLP investors? You bet. After two decades of wiping the exchange floor with stocks, bonds and every other asset class, the strong track record of MLPs is not only not lost on any buyer but probably provides much of the motivation. So far, with the notable exception of 2008, the buyers have been nearly always right to follow the trend. As you’d expect, this trend following has created some facts on the ground that defy almost any other explanation.

To get specific, look at the strong recent outperformance of low-yielding growth MLPs. Refinery logistics spinoff Phillips 66 Partners (NYSE: PSXP) is now up 230% since its initial public offering not quite a year ago, and 25% since the end of May. Its yield is down to 1.4%, well below the 2.3% for its much cheaper parent Phillips 66 (NYSE: PSX) and less than a quarter of the 6% on offer from Holly Energy Partners (NYSE:HEP), an older and equally reliable refinery logistics MLP.

Since investors are clearly not buying PSXP for its current yield, they’re obviously after the promised high rate of distribution growth based on expected asset dropdowns from PSXP’s parent. Buying on that premise assumes that the record-low current yield will stay down, because otherwise the unit price will not receive the full benefit of the projected distribution growth. But every buyer in the financial markets presumably expects the price of the purchased security to rise. The fact that so many still expect that from PSXP after the run it’s had and despite its very modest payout seems instructive about the current sentiment among the buyers of MLPs.

Can this process keep going? Absolutely. Reflexivity is not just about the buyers making everything too expensive. By their buying, they’re also making other changes in industry fundamentals. For example, expectations of asset dropdowns from PSX to PSXP become more rational the more the MLP outperforms its sponsor, since the unit price than creates financial incentives for the parent to transfer the assets to its pricey affiliate with its dirt-cheap capital.

If only this virtuous circle could run forever, the triumph of PSXP’s current buyers would be assured. Unfortunately, PSX doesn’t have unlimited assets and for that reason alone PSXP’s rate of distribution growth is destined to slow, at which point the market will stop expecting capital gains and start seeing an inadequate yield not offset by the declining growth rate. It’s not clear who’ll want to buy PSXP then, but history tells us reflexivity is a very harsh mistress once she tires of her latest object of affections.

The thing to keep in mind is that PSXP’s story is just an exaggerated version of the long-running tale of MLP exploits. It seems obvious that reflexivity is in play in unit prices, in the heaping pile of MLP investing vehicles seeking to capitalize on the track record of this asset class and in the enthusiastic pace of asset dropdowns and sales to MLPs by the considerably less expensive corporations.

That obviously hasn’t hurt anything to this point and reflexivity never does, at first. In fact, the building investor demand provides cheap capital that can fuel future investments, in an industry that given the domestic drilling boom has no shortage of lucrative investment opportunities at the moment.

But the shale fracking revolution is hardly news to anyone at this point, and unless you believe its fundamentals have improved 15%+ since the beginning of the year you must give reflexivity its share of the credit for the recent gains.

Unless some of this trend-following bullishness is dissipated over time, a peak and then the unwind of the reflexive buying could some sooner than later. At that point, the conviction of the recent buyers of the lowest yielders about those bullish long-term fundamentals could be tested like it hasn’t been since 2008. I fear they’re not prepared for what comes next.