Many reasons! You may have gotten hot and sweaty in your boots and then had the sweat cool. Your boots might have leaked water through. They may be too tight so they’re cutting off circulation. You may have the wrong type of socks.
Good boot fit is one of the most important parts of snowboarding – you want them to be tight but not uncomfortable, and cushy inner soles can keep you snowboarding in comfort for longer. Also, don’t wear two pairs of socks. You’re better off with one decent pair as it leaves more room for a warm air pocket to form. Your toes will thank you.