Burnside requested said leave. He was not "placed on leave."
This appears to be an attempt to rely on a technicality to avoid noticing what was going on.
Burnside had been in hot water ever since The Crater on July 30. This is obvious from reading the messages back and forth from reading the messages in the OR.
On the evening of August 13th, the "leave" came about as a result of a Grant-Burnside meeting of the minds. It was not a routine process -- because if it was Meade would have known about it. Instead, the first Meade and his HQ know about it is when General Wilcox notifies Meade he has assumed command from Burnside. As far as Meade knows, this is when Burnside goes on "leave":
HEADQUARTERS ARMY OF THE POTOMAC,
August 13, 1864-5.15 p.m.
General BURNSIDE:
Your dispatch of 4.45 p.m. has been submitted to the major-general commanding, who directs that the additional requisition of 400 men must be furnished. If a serious attack should be made, the details can quickly rejoin their commands.
A. A. HUMPHREYS,
Major-General and Chief of Staff.
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HEADQUARTERS NINTH ARMY CORPS,
August 13, 1864--9 p.m.
Maj. Gen. A. A. HUMPHREYS,
Chief of Staff:
GENERAL: I have the honor to report myself in command of this corps, and nothing important has occurred on the lines during the last twelve hours.
O. B. WILLCOX,
Brigadier-General.
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HEADQUARTERS ARMY OF THE POTOMAC,
August 13, 1864--9.40 p.m.
Brigadier-General WILLCOX,
Ninth Corps:
Your dispatch is just received. The commanding general desires to know by what authority or under what circumstances you are in command of the Ninth Corps, as nothing to that effect has been received by him,
A. A. HUMPHREYS,
Major-General and Chief of Staff.
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HEADQUARTERS NINTH ARMY CORPS,
August 13, 1864--9.55 p.m.
General HUMPHREYS:
GENERAL: In reply to your dispatch of 9.40, this command was turned over to me this evening by General Burnside, who goes away under a leave of absence for twenty days, granted, as I understand, by Lieutenant-General Grant. I supposed, of course, it was known by the major-general commanding the army.
O. B. WILLCOX,
Brigadier-General.
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HEADQUARTERS ARMY OF THE POTOMAC,
August 13, 1864--10 p.m.
Lieutenant-General GRANT:
Brigadier-General Willcox informs me he has assumed command of the Ninth Corps. No official communication has been received at these headquarters authorizing General Burnside to leave. Has any action been had or taken?
GEO. G. MEADE,
Major-General.
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HEADQUARTERS NINTH ARMY CORPS,
August 13, 1864--10 p.m.
Major-General MEADE:
I have turned over the command of this corps to General Willcox, having received permission from Lieutenant-General Grant to absent myself from this post. I supposed you knew of it.
A. E. BURNSIDE,
Major-General
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HEADQUARTERS ARMY OF THE POTOMAC,
August 13, 1864--10.15 p.m. (Received 10.20 p.m.)
Major-General BURNSIDE:
Your dispatch to the major-general commanding is received. He requests me to say he had no notice of your authority to be absent, but presumes it is all correct.
A. A. HUMPHREYS,
Major-General and Chief of Staff.
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CITY POINT, VA., August 13, 1864.
Major-General MEADE:
I gave General Burnside leave of absence. The leave is only just made out this evening, and directions were given to communicate the fact to you, and no doubt it would have been communicated in the morning by the assistant adjutant-general. General Parke will command the Ninth Corps. He has gone out this evening.
U.S. GRANT,
Lieutenant-General.
As you can see, at 4:45 PM Burnside is still handling routine work with Meade's HQ (this is about supplying a work detail of 400 men for Captain Harwood). At 9 PM, General Wilcox is reporting to Meade's HQ that he is now in command of IX Corps -- which surprises the heck out of Meade and his Chief-of-Staff Humphreys.
In between those two points, Burnside talked with Grant, "applied" for a leave of absence, Grant "gave" it to him, Burnside turned his command over to Wilcox, and Wilcox promptly notified Meade's HQ that he was now in command. All of that happened in what looks like
four hours.
You seem fairly well aware of military procedure, so you surely know that is not normal. A court of inquiry had been convened to investigate The Crater situation (at Meade's insistence) and was in sessions from August 6 to September 9. Burnside was one of those being investigated. Suddenly on the evening of August 13th, Burnside is placed on leave in a very unusual way that surprises Meade, Humphreys, Wilcox and probably a bunch of others.
Maybe Burnside asked Grant for a leave, maybe to prepare for his testimony to the board of inquiry and possible court-martial board. Maybe Burnside felt he could not be an effective leader in that situation and offered to step down. Maybe Grant told him that Burnside could not be an effective leader in that situation, then asked Burnside to submit a request for leave. Maybe there was a Grant ultimatum or pressure involved; maybe Grant just wanted Burnside to go and Burnside agreed.
No matter what, though, Burnside was stepping down because he was under pressure. Pitching it to the public as a "leave" is just what would be called "spin" today.