Reading between the lines on Hansard (not always the most sensible thing), I would guess that the Territorial and Reserve Forces Act 1907 was wot dun it. The Lord Raglan no less speaks on 16 July 1908 about the act and says:
"We all know that an immense and most dangerous reduction has been made in the Regular Forces by His Majesty's Government, but it is amazing that quite recently the War Office have closed almost the whole of Section D. of the Army Reserve. I should like to ask the noble Lord the Under-Secretary of State for War why Section D. of the Army Reserve has been closed. It is most remarkable that the War Office seem determined, whenever it is in any way possible, to reduce the number of His Majesty's soldiers. If they can issue an order, such as that closing Section D., which will affect very large numbers of men, that order is issued; if they can, by any twisting of regulations, issue an order which will discharge men, that is done; and the result is that the Regular Army, as regards both officers and men, is in a state of commotion and turmoil, and in a great number of cases in a thoroughly dissatisfied condition". The answer broadly was that there were enough reserves and that the closure was just a temporary measure (!!).
Although this refers to Section D, it does suggest that the Act reconfigured the reserves completely.
May help to narrow down the search?