To the best of my awareness Chaos it is completely random unfortunately. At least that seems to be the majority consensus anyway. However, I have noticed the same thing, and a few other peculiarities too. I have (well had) three bases myself. They are:
- A T1 sandstone tower, 3 storey, on the newbie river. This was my first house.
- My second and main base. A T3 black ice compound, two storey. Situated on the grassy area between the River and Jungle.
- A recent addition. A small sized T3 reinforced stone hut in the Unnamed City. This was mainly just built to test a bug in a previous thread.
Strangely enough, the only base which has ever been purged for me so far was the Sandstone one (my first one). Even after I moved most of my stuff to the black ice cimpound. Strange, as I expected the bigger, higher tier base to be the more likely target. This is pure speculation here, but I am actually starting to wonder if there is a subtle bug which ties them to a certain base. Chaos, is the one that keeps getting purged on your game the first base you built or not?
If there is a particular base you want the purge to appear at, my best advice at this point in time (which is hardly proven) would be as follows. Try to increase the ‘hub’ factor there. Ie- try building it bigger, using some more higher tier materials, add more thralls, a shrine, crafting stations, and more placeables. Also try storing a good reserve of resources (stone, wood, bricks, etc) there too, be they high grade or otherwise. This has been my approach so far, but the results are too preliminary, and I still need more time to test this. I can keep you posted on the results if you would like.
In the meantime Im not sure if you happened upon it yet or not, but there is another short thread exploring this issue, which you can read for some additional information if you like. It is: