What’s happening here is that you’re placing a pointer to a list in the hashmap, not the list itself.
When you define
List<SomeType> list;
you’re defining a pointer to a list, not a list itself.
When you do
map.put(somekey, list);
you’re just storing a copy of the pointer, not the list.
If, somewhere else, you follow that pointer and modify the object at its end, anyone holding that pointer will still be referencing the same, modified object.
Please see http://javadude.com/articles/passbyvalue.htm for details on pass-by-value in Java.