more to do with the French needing pressure taken off them at Verdun.
they were getting slaughtered holding out against wave after wave of German attacks and the French politicians begged the British to make an attack somewhere in order to divert German troops to stop that British advance, thus taking the pressure of the French at Verdun. The British were worried that the French lines would collapse and if that happened they would have been encircled by the Germans.......which is not good. Thus the British, under pressure from the French agreed to attack. Haig had said his troops, mostly conscripts, were not ready but basically followed orders.