I agree with the long-term prospects of ACB. However, the reason for the high volume is because ACB is being TRADED. The "Traders" are not investors.
When a stock is hitting its all time high - that is when the "traders" start shorting the stock. When they short, they sell borrowed shares and the relentless selling will drive the stock lower.
So when ACB is flying-high, the shorts are out in groves.