The reason why people started dancing during the dancing plague in 1518 is due to a result of extreme stress, mass or mass psychogenic illness and strong cultural beliefs in curses or divine punishment, which lead to a collective psychological breakdown, in which anxiety manifested as uncontrollable physical movement, which sometimes was also triggered by witnessing other people dance.
The severe conditions of famine, disease as well as societal upheaval in Strasbourg at the time also intensified the distress, which caused people to dance it out during the dancing plague, which worked as a physical release of pent up fear and trauma.