An injury to a testicle can affect the testosterone in the deer, the levels of testosterone in the deer tell the deers body when its time to shed its antlers. Im not completely confident the way i explained it will be clear but im going to bet the deer either injured a testicle or lost one last year and it wo t lose its velvet this year and possibly keep the old horn, same reason you see some bucks killed late in the year that are still in velvet not bery common at all but it does happen