It also requires proximity to base structures in order to trigger, so if you are spending a considerable amount of time away from bases they don't get raised. I had gotten a couple "Forest is Moving" raids before beating the Elder, though I do spend a lot of time around the base doing farming and such plus smelting.

The strangest thing was getting a Swamp raid one day after resting after clearing a crypt and prepping to haul the iron back by cart, forgetting the materials for the cart, after I had already moved the portal to a larger swamp some distance away. Then on the trip back the next day with the cart, and I triggered a Ground Is Shaking raid in the middle of the road route between my outpost and my main base. I believe I had a roadside outpost from when I built the road that would have had a crafting station, fire, and maybe a bed?? (Pretty much destroyed when I ran away:) I was surprised a raid was triggered.

It's worth noting that the raid countdown only continues while you are within the radius of the raid. I had left the radius with trolls on my tail, then was waiting for the event to end, which after several minutes was still on the map. I wasn't very far away, still visible in the mini-map area. It wasn't until I would return and stayed in the area that it seemed to end