Thieves are normally thought of as, “One who steals/ steals with [aspect] for their own benefit.” Thieves are the self-oriented opposite of Rogues.
Mind is all about mentality, rational thought, and decisions, and how those decisions affect the individual who made the decision, or the people that decision would affect.
A Thief of Mind would steal others ability of rational thinking, and steal their ability to make good decisions, and keep that for themselves.
Hope that helps!
~S.