Словно не хватает такого же события на начале операции перемещения (Drag)
Посмотри события у DocumentCollection
using Cad = Autodesk.AutoCAD.ApplicationServices.Core.Application;
DocumentCollection docMen = Cad.DocumentManager
DocumentLockModeWillChange, после него наступает событие.
DocumentLockModeChanged
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg Write, Write => NotLocked GlobalCommandName:GETCLIPDATA
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg NotLocked, NotLocked => Write GlobalCommandName:#GETCLIPDATA
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg Write, Write => NotLocked GlobalCommandName:DRAGENTER
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg NotLocked, NotLocked => Write GlobalCommandName:#DRAGENTER
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg Write, Write => NotLocked GlobalCommandName:DROPGEOM
DocMen_DocumentLockModeChanged (DocumentCollection, DocumentLockModeChangedEventArgs) Чертеж1.dwg NotLocked, NotLocked => Write GlobalCommandName:#DROPGEOM