Answer (With thanks to Nicholas Mansfield): At the beginning of the war, part-time regiments of the Territorial Force, were each asked to raise a separate second line unit, as part of the enormous expansion of the forces. (Volunteers for Kitchener’s New Armies were in a separate parallel development.) Pre-war Territorial soldiers only had to sign up for home service and though at the war’s outbreak a majority volunteered for overseas duties, a sizeable minority did not, so these individuals were transferred to the second line unit which were stationed in the UK for training duties. Examples include the 6th Suffolk Regiment, 6th Royal Sussex Regiment and 1st Cambridgeshire Regiment where 40% opted to stay at home. It was even higher in the county Yeomanry – the mounted part of the Territorial Force, which also raised a second line. Consisting partly of rural landowners and farmers’ sons, these did not want to leave their agricultural holdings and generally stayed working at home during the war.
To complicate matters Territorial regiments also briefly raised third line units (e.g. 3/4th King’s Shropshire Light Infantry) but these were quickly absorbed into centralised training and reserves and lost their local connection.