The Hub had a lockdown system. By the time of Torchwood Three's re-establishment, it was only semi-functional and Ng was unaware that they still had it.
It was activated by a firework that Yvonne Hartman set off and claimed was a glitterbomb. Orr and Jack Harkness both looked into undoing the lockdown, which would take a long time. (AUDIO: Poker Face)