From what I've read on Twitter it sounds like people were "evacuated" through security from landside to airside, or there was something linked to the evacuation that caused a security concern. The comments weren't completely clear but the impression I get is that people got airside without being screened, therefore the departures/airside areas are no longer sterile. It's then a case of everyone out, security and dogs in, and the entire airside area having to be searched that can take a long time.