Spontaneous

1. Self-generated; happening without any apparent external cause.

2. Done by one's own free choice, or without planning.
3. Proceeding from natural feeling or native tendency without external or conscious constraint
4. Arising from a momentary impulse
5. Controlled and directed internally; self-active; spontaneous movement characteristic of living things
6. Produced without being planted or without human labor; indigenous
7. Sudden, without warning.

(self-generated): autonomous

(done by one's own free choice): autonomous

(proceeding from natural feeling...): autonomous

(sudden, without warning): abrupt, precipitous, subitaneous; sudden