Carlisle Parade has remained relatively unchanged in the years since this military parade took place. The question here is what is the military parade for? The original photograph was in the Hastings Forum, which is part of the 1066 Online website. Contributors there suggest it took place to mark the end of the Boer War, in which the 1st Cinque Ports Rifle Volunteers took part.
At that time the Cinque Ports Rifle Volunteers had adopted blue slouch hats with a blue plume upon their return from South Africa, and would still have been dressed in red tunics as shown here.
Through my work with the Hastings & St Leonards Pictorial Advertiser, I have found the answer. The photo is from a medal presentation ceremony by the Canadian Army, who were stationed in Hastings in February 1917. Further on in this post is copy of the article presented in the paper. It is likely to be the presentation of the Military Medal to Private W F Ede of the 52nd Battalion, Canadian Expeditionary Force. He was awarded the medal for carrying a wounded comrade away from battle whilst under fire.