Security is a top priority in the air travel industry. The federal government, through its various agencies, has strict regulations about whom and what they allow beyond the airport security gates, out on the tarmac and on commercial airplanes.
Security personnel take extra precautions when threats seem imminent. This includes suspicious occurrences such as an unlabeled box found abandoned in the terminal or an unidentified person found roaming the tarmac. The airport could also be on high alert after a bomb threat.
Depending on the threat, extra security precautions might include grounding flights scheduled to depart or stranding arrivals on the tarmac rather than allowing them to approach a gate. In extreme cases, airlines might divert your flight to another airport or cancel the flight altogether before you take off.
One of the most dramatic impacts on flights due to security precautions occurred in September 2001. That month, which included the four-flight terrorist attack that shook the United States, airlines canceled 99,324 (20 percent) of their scheduled flights. This anomalous event impacted statistics (and travelers) dramatically that year, with twice as many flights canceled than in any of the nine years that followed. [source: RITA]