This chart shows what could really be behind the crazy run in pot stock Tilray

  • "There is not much stock left to borrow in this name," says Ihor Dusaniwsky of S3 Partners. Fees for borrowing Tilray stock were ranging between 500 and 700 percent, making it an incredibly expensive stock to bet against.
  • Dusaniwsky also noted investors long the stock are not selling out of their positions and are "salivating at the windfall of lending revenues in this name."
  • Data from S3 Partners show that short interest in Tilray has stabilized around 3.5 million shares from about 2.5 million shares late last month.

There may be a technical reason for the magnitude of the rally in Canadian pot producer Tilray, which has captured the attention of traders everywhere: It's too expensive to bet against the stock.

"There is not much stock left to borrow in this name," said Ihor Dusaniwsky, managing director of predictive analytics at S3 Partners, in an email. He noted that fees for borrowing Tilray stock were ranging between 500 and 700 percent on Thursday, making it an incredibly expensive stock to short.

(Short selling is the practice of borrowing shares of stock for a fee, then selling the shares in the hope of buying them back later at a lower price.)

Tilray shares started to rip higher about a month ago. Short interest on the stock also spiked to about 3.5 million shares — approximately a third of the company's overall float — from roughly 2.5 million shares in that time, S3 Partners data show. That means more and more investors were betting against the name.

The stock just went public in July and there are relatively few shares available to trade. The chart below shows that at some point along the way, it became just too expensive for short sellers to borrow the stock so they could bet against it.

That also happened to coincide with the big leap forward by the stock this month.

CNBC | Getty Images

Source: S3 Partners

Short interest has remained stable around those levels for most of September while Tilray's share price has continued to surge.

Dusaniwsky also noted investors long the stock are not selling out of their positions in part because they could potentially make so much money lending their shares to short sellers. They are "salivating at the windfall of lending revenues in this name," he said.

S3 Partners data show Tilray shorts are down more than $700 million in mark-to-market losses for 2018, with $608 million of those losses coming this month alone.

Tilray shares ripped 56 percent higher in the previous two days, but with plenty of bumps along the way.

The stock was driven 38.1 percent higher on Wednesday after CEO Brendan Kennedy told CNBC's Jim Cramer that global pharmaceuticals must think about partnering with cannabis producers as a "hedge" against the space. On Tuesday, the stock surged 28.9 percent after Tilray announced the Drug Enforcement Administration approved it to import marijuana to the U.S. for medical research.

Trading in the stock was halted five times on Wednesday, however, as it failed to hold a more than 90 percent gain. On Thursday, Tilray shares dropped more than 15 percent.