Why does pop() removes an element from more than one list?

[buran]

I made list2 a copy of list1.

Actually list1 and list2 point to same object. the assignment just copies the reference to the list, not the actual list, so both list1 and list2 refer to the same list after the assignment.
to make a copy of list use

list2 = list1.copy()

or

list2 = list1[:]

you can also use copy.copy() and copy.deepcopy() from copy module