it was a phrase coined by the late Rudyard Kipling, referencing predominantly English Gentlemen in India during the main period of Imperial rule, specifically those English gentlemen who would take their dogs out hunting the local wildlife for sport, particularly the Bengal tiger, it was noted that these English gentlemen were 'mad' to go out during the hottest part of the day, and while that is a true enough statement it was learned years later that it also had to do with the behavioural change seen in the hunting dogs due to severe heatstroke making them act in a 'mad' way, running around in circles, not obeying commands, and frothing at the mouth and gnashing their teeth, the phrase was made popular by the song of the same name by Noel Coward, it was this specific behavioural change that the Orcs are exhibiting, and through word of mouth like an old wives tale they as a species have been indoctrinated to fear the sun because of the 'madness' and 'fear' it can cause them due to exposure.
i feel like quoting Aslan here, because i'm sick and fed up of having to explain the point being made to multiple people multiple times because they are incapable of understanding said point from the information provided to them, in future, don't try and sound like a smart arse it doesn't suit you.