Without knowing exactly what the paperwork states in Major Newell's record concerning 2nd Sergeant John Mort, Company A, 10th Virginia Infantry, it is, of course, impossible to identify why such document was filed in Major Newell's record.
However, there is one likely explanation.
Major Newell was a highly respected and regarded individual in/and around Strasburg. Sergeant Mort was a member of the Strasburg Guards, Company A, 10th Virginia. It is very possible the paperwork in Major Newell's file is in regard to his obtaining whatever pay may have been due Sergeant Mort's widow and children. Sergeant Mort was killed in action 29 August 1862 at the Battle of Manassas No. 2.
If you could either send me a copy of the paperwork found in Major Newell's file, or post what is contained in the paperwork, the exact reason may be able to be determined.
As an aside, John Mort's brother, Jeremiah served in Company C, 136th Virginia Militia. Later he enlisted in Company E, 11th VA Cavalry and was killed in action in June or July 1864.

John P. Mann IV
199 Towers Rd.
Mt. Solon, VA 22843