Both Uses

spontaneous
in
Unbroken, by Hillenbrand
(Auto-generated)

Late one night, Louie climbed the steeple of a Baptist church, rigged the bell with piano wire, strung the wire into a nearby tree, and roused the police, the fire department, and all of Torrance with apparently spontaneous pealing.†
p. 7.5

spontaneous = behaving in an instinctive, uninhibited manner OR happening naturally (without planning or external force)
Louie noticed that when he drank the stuff, his chest hair spontaneously fell out.†
p. 90.4 *

spontaneously = happening in a natural manner without planning or external force

Definitions:

(1) (spontaneous)
happening naturally, suddenly, or without outside planning or action
(2) (meaning too rare to warrant focus)