Assalamualaikum Warahmatullahi Wabarakatuh

In the name of Allah, the Entirely Merciful, the Especially Merciful. O Allah! I seek refuge in You from knowingly associating partners with You in any way, and I seek Your forgiveness for that which I do not know. Ameen.

Why is the city of Medina called the Haram Land? 

Allah s.w.t. made Medina is an haram city just as Allah s.w.t. made Mecca also as an haram city.  

And what is meant by Haram Land? 

The Prophet Muhammad s.a.w. said the following: 

Allah bless you "Indeed, Abraham made Mecca a haram land, and indeed I made Medina a haram land," (HR Muslim). 

The meaning of haram here is that it is forbidden in the cities of Mecca and Medina to cut standing trees, kill game animals, and raise weapons for the purpose of shedding blood or fighting. The Prophet Muhammad s.a.w. said:
"God willing, God willing, God willing, God willing, God willing. In fact, I forbid cutting standing trees and killing game animals in Medina."  (HR Muslim). 

In another history, the Prophet s.a.w. said:
"Indeed, I forbid the city of Medina, whose territorial boundary is between two mountains in the city of Medina, from shedding blood, not bringing weapons to war, and not dropping the leaves of the trees in the city of Medina, except for animal feed." (HR Muslim) 

Thus, this is the real meaning behind calling Medina and Mecca the Haram Land.

Wallahua'lam.