The event is biological in origin, with someone or multiple people developing or discovering a power unique to their body. Some examples and ideas are:
Someone finds out they are a bodyhopper, able to possess or swap with other people. Potential ways of possessing and swapping are: Through touch, by turning into liquid/smoke/particles or something else, through sex, and other ways.
Someone awakens the ability to take other bodies into their own, through fusion, merging, absorption, or similar ideas. This person is only able to do this as long as their own body is involved, unable to do it remotely with two others.
Someone becomes able to turn their body and soul invisible to common people. They are then able to share the power with others by dematerializing them as well. While dematerialized, they can fly through walls and possess others, but not affect non-organic things while ghostly. Possessing others, they can memory wipe them, keep their memories in place knowing that they were controlled, make them think that their actions were their own, and influence their minds but leave them in control.
For some reason, almost everyone in town develops unique powers around the same time. Some powers are invisibility, telepathy, becoming irresistible to others, possession, body swapping, merging/fusing/absorbing others bodies and their powers, mind control, time stop, personality change/swap, shapeshifting, and many others.
Other ideas?