Also happens during war, when infra losses reduce your citizen count. You keep your improvements already bought, but your available improvement slots will be reduced accordingly, even into negative count.
In my nation's earlier days, I used to get hit with 3 or 4 negative events simultaneously. Recently, though, I've been getting the good ones. It's still totally random, though.
Here's the link to the old IF forum:
http://z15.invisionfree.com/Cyber_Nations/...showtopic=60043
The forum's up again, although it's read-only now.
B)