Think it's because EVERYTHING is half price or less, straight away, compared to most shops 20% off, gradually going to 50-60% over the next month.
I worked in Next before Zara, and we would have people queueing from 4-ish when we got there to put the tills in. If you want to shop Next, have a plan. If you need homeware, you need to be there for opening, that's what goes the fastest. Then move to kids, ladies accessories, ladies clothes, and finally mens. If you have nothing in particular that you want, but just 'bargains', the shoppers seem to come in waves. First one at 5, next one 7.30-8, think it's as the kids wake up, then starts getting busy again 10-ish for the rest of the day. So best point to hit would be just gone 6, it's not manic, but there's still stuff left. If you need homeware, get there first thing, prepare to use elbows, and just pick up everything you think and put back what you don't want, as it won't be there when you go back.