Dollar Tree (Nasdaq: DLTR US) shares closed 14,3% higher on Monday, to a record intraday high, after Dow Jones reported that activist investor Mantle Ridge has taken a $1.8 billion stake in the discount retailer, prompting multiple analysts to upgrade the stock. It is easy to see why Dollar Tree has been a target of activists. Since 2016, the first full fiscal year with Family Dollar, Dollar Tree's revenue has grown at a compound average growth rate of 5.3%, while competitor Dollar General's revenue has increased by 11.3%. It is obvious that Mantle Ridge will put DLTR’s management under strong pressure for changes.