Because that is what the default ToString overridde of ListViewItem does: Prints the name ofg the class "ListViewItem" followed by a colon, and then the data content in curly brackets.
ListViewItem lvi = new ListViewItem("hello");
Console.WriteLine(lvi);
Will print
ListViewItem: {hello}
I assume that you have removed a class which derived from ListViewItem, and which overrode ToString to produce the output you wanted.