Special base attack events, like "Eikthyr rallies the creatures of the forest," should trigger at randomized intervals whenever a player character is within a certain range of certain base structures. I haven't calculated a mean time to happen, but if it's been a long time, that's just luck, or perhaps you've been out exploring more than normal.

Relocating your home base won't affect this at all - wherever it is, when you're there, the base attack event may trigger, and if it does, you'll get hit by whatever the current event is (based on whatever boss is next for you to fight).