Give the people what they want...

So i'm playing as the Saxons, we're into the 370s, and my capital isn't happy. I check out the city details scroll, and i've got massive unrest and blinking trade icons and all that good stuff.
I click on the only available trade building (shipwright) to put it into the build queue, as I want to see what affect this will have on my cash flow.
When I do this, all of that unrest disappears, my people are happy as clams, and not just in my capital, but in all of my provinces.

Why did this happen? Do the Saxons have some compulsion where they MUST have a shipwright in their capital or something?

It could have been due to the fact that your christian, faction leader dies and a new, pagan one takes over the lead. Most of your settlements is probably pagan, so this`ll remove 10% unrest in every pagan settlements. If your faction leader died in a battle that turn, the effects won`t show up before you add a building or a unit to the construction/recruitment queue. This is one of many possibly reasons.

Since v1.2, at the start of every turn, there's often more unrest reported in a settlement than there really is. Adding something to the build queue (and then removing it if necessary) takes away the excess unrest. It doesn't matter what you shove in the build queue, the excess unrest disappears for that turn. That doesn't mean all unrest goes though.

Perhaps that's just something that happened to you.

Dunno my Saxons are 100% Pagan and 100% happy. Only faction so far that has given me no religious unrest.