Wasps are sensitive to noise and light
Wasps are very sensitive to noise and vibrations and they can quickly detect the odours of human presence, particularly if the person is nervous.
Once disturbed, the whole colony may go into an emergency defensive mode, attacking anything that moves nearby, hence certain well documented news stories of people being attacked and going into anaphylactic shock after repeated stings.
Something as simple as cutting your grass near or banging at the entrance of the hole to their nest, can lead to an abundance of wasps reverting to their defensive mode.
Sometimes they can be effected by artificial light such as a flood light or torch. They see the light as daylight and most likely will emerge from their nest ready to work. This is one way to see if a nest is still active. If you shine a torch at the entrance from a safe distance and wait... they will fly out if it is still alive.