Shares of Cronos Group Inc. /zigman2/quotes/206842762/composite CRON +0.77% /zigman2/quotes/202715342/delayed CA:CRON -0.66% rampaged in the extended session Wednesday, gaining as much as 41.5% as of 6:22 p.m. Eastern time on heavy volume, before dropping to a 33% gain at 6:45 p.m. Eastern time. It was not immediately clear why the stock surged in the extended session but two block trades totaling roughly 1.2 million shares crossed at 5:09 p.m. Eastern time at a price of $8.40 a share. If Cronos shares hold on to the gains, the late Wednesday move will be the second largest in the company's history and the largest since the company began trading on the Nasdaq. Previously other cannabis companies' stocks, such as CannTrust Holdings Inc. have notched double-digit gains because an exchange traded fund has re-balanced its portfolio after significant changes to the market price. Cronos Group stock closed up roughly 1% to $8.40 during the regular session. The stock was up 14% in premarket trade Thursday.