BTP probably didn't want them in Brighton station until the rush hour crowds had died down.
The biggest mistake about this game, imo, was playing it on a weekday evening. It should have been a Sunday lunchtime kick off, like the St Patrick's day game.
According to reports, they threw a flare at the turnstiles, which distracted the stewards, police and sniffer dogs, whilst some already inside opened a fire exit, so getting in via two different methods.