The torah (27;17) states: "Accursed is one who moves back the boundary of his fellow. And the entire people shall say Amen."
However, what's so bad about moving your friend's boundary that it's deserving of a curse?
Rashi (27;17) explains that when one moves their friend's boundary they transgress the sin of stealing.
However, the problem arises: The torah (19;14) relates the same case ("You shall not move back the boundary of your fellow") and yet Rashi over there (19;14) states that one transgresses the sin of robbery for their actions. What sin does one transgress for moving their friend's boundary: stealing or robbery?
In order to answer this question we must understand: What's the difference between stealing and robbery?
Rabbi Yochanan Zweig explains that stealing is when one takes from his friend and he's not around, whereas robbery is when one takes from his friend in front of him.
Now, the question arises: If the person being stolen from is going to know exactly who stole from him afterwards then is the one who took from him considered to have stolen or robbed? According to the Rambam the answer to that question would depend on if owner saw the act while it was happening or not (if yes-robbery, if no-stealing). Therefore, if one knows who stole from them then the sin should be robbery (not stealing).
However, if that is the case, then how could we ever have a case of stealing?
Rabbi Zweig answers that we have a case of stealing when no one is ever going to find out what was stolen. For example, if one moves his friend boarder very slightly then the owner is quite possibly never going to realize his loss of land. In contrast, the definition of robbery is when the person is going to find out what was stolen from him.
In closing, Rabbi Zweig explains that when one steals from their fellow when they aren't around and assume that they will never get caught they show that they are only concerned about their self image. However, we must all learn to do the right things because they are right--not out of concern for what others will think of us. Based on this, Rabbi Zweig explains that the curses here are for people who are comfortable doing things in which no one will ever find out about (I.e. stealing). One who steals from their friend takes no responsibility for their actions as they think that they are going to get away with it. As a result, one who steals gets cursed because they show that they values their self image higher than the need to do what's correct and should be done.