It's a Concert, Not a Campsite
Well, it might get chilly, you never know — so why not pack a sweater, and maybe a coat while you're at it. And since that means you're bringing a backpack, might as well stuff in some snacks and also some band paraphernalia in case you somehow get backstage for signatures. Add a book for the bus/car/train ride and some water and rain boots, and suddenly your pack is stretched to capacity, extending straight out from your spine. Luckily it's high-tech, so with the belt and chest straps and ergonomic cushioning you barely feel the weight, and it doesn't slow down your interpretive dance moves.
Strange that there's nobody standing near you anymore. Even your friends are keeping their distance. Maybe it's because every time you turn around you unwittingly deck the person next to you with your enormous backpack.
If you're not at a multi-day festival, leave the luggage behind. Even if you are, maybe think about stashing your enormous pack in your tent. When you're in a crowd it's best to be as streamlined as possible — for your own sake and that of others.