Honestly I have been trying this since quite a while and with a group and alone. it just does not appear at all anymore. As far as I saw it at least.
The event appears after you have defeated the rockspitters at least 3 times.
Oh, okay. Thanks. Cause I had been lucky and got out of the cavern after only fighting them once.
Thank you very much. Also hope for more content soon. I really enjoy this game. Hehe