The Wall Street Journal reported in July that short sellers, investors betting against the stock, had increased substantially in recent months, and at the time represented 16% of total outstanding shares.

In any event it is likely that Chipotle has until February to make some changes -- otherwise there is a real possibility the burrito chain will find itself in the crosshairs of a proxy contest. According to a Chipotle securities filing, shareholders seeking to nominate a dissident slate of directors for the 2017 annual meeting must do so between Jan. 11 and Feb. 10

The campaign comes after Ackman last month liquidated its remaining 6.6% stake in Canadian Pacific Railway (CP) , the fund's latest effort to unload shares in the wake of losses stemming from its large minority investment in embattled Valeant Pharmaceuticals International (VRX) .

Despite the Valiant struggles, a new activist campaign by Ackman may not be a surprise. Ackman said in a press release in August that the fund intends to use the proceeds from the liquidation of the CP stake to allocate towards one or more new investments.

CP announced Tuesday that Ackman has resigned from the board of the railroad operator, bringing to a conclusion a five-plus year campaign at the railroad company that had its ups and downs.

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