Why did Sarawak surrender?

[Chernobyl]

In my game messing around as Japan, Sarawak unexpectedly surrendered to Japan. I didn't capture the capital Kuching. I did conquer Brunei and the DEI, and I captured the oilfield Miri too, but that's it. Sarawak just decided it's time to give up. Is there a known reason for this? The manual only says surrenders happen when capitals are captured, 0% national morale, or some scripted event possibly. GB has 95% NM so it isn't that.

There is a Dutch cruiser hanging out at Kuching port that is now in a pickle, if that could have anything to do with it.