Imagine you are running a prosperous long-term fortress, with a growing population and a not excessive rate of death. Your reader/expedition leader/social dwarf becomes your mayor in time, and is probably elected many times over. Things are pretty cool. You get attached to him, and he ends up one of your more experienced and highly valued dwarves.
Assuming your fortress survives and prospers, you attract nobles. Your count, baron, or duke, and his/her consort, show up and officially take over the fortress - although your popularly elected mayor is still there, so that's cool.
Now, what do the ruling nobles do, other than mandate impossible-to-manufacture goods? They pump out babies. A noble and consort can produce at least a baby a year, sometimes more. Few other dwarves in the fortress can match their fecundity, since even haulers are usually too busy to have quite that many kids. Military dwarves breed, but their children have a somewhat higher attrition rate than those of nobles.
A noble's children follow her everywhere, a noticeable swarm of little red smiley faces. As they do they socialize with each other, the other nobles, and anyone else who pauses to take a break. This usually results in them all being friends with each other, as well as quite a few other dwarves. When the first one of those noble spawn reached adulthood, their massive number of family and friends usually cause them to win the next mayoral elections. From then on, your mayor will be a child of the nobility. It may change from one to another over time, but there is little you can do to give any other dwarf a chance to beat the noble's voting block. Seems to me to happen about thirty years into a healthy fortress.
I saw this happen in my fortress, but it didn't really occur to me what was happening till I re-read the story of Nist Akath and realized this was how Captain Ironblood was finally replaced as mayor.
The obvious answer most of you will suggest is burning the nobles in lava. While this is always hilarious, you'll have to do it every few years, and it will kill your immigration rate. You could merely kill the consort, who won't be replaced and will prevent the nobles from developing a family voting block. The damage is already done in my fortress's case, although my recent experiments in suspended animation for troublesome nobles are yielding promising results.