How many times has that come across your HUD... (or how many times have you caused it to appear on the HUD of others?)
One of the first things a new player starts receiving is offers of gold to come join someone else. It's probably the most common automated event in the game. So...
why should you accept that bribe?
Particularly for new players who don't have a full understanding of what is happening, this is often the case. Say yes and see what happens. Well, what does happen? Your kingdom may change. Your liege's name certainly changes. And that's all of the differences that you see on your hud. However, your lieges- past and (new) present- see your acreage either appearing or disappearing from their total holdings... and will be either happier or sadder (depending on which side of the change they're on) because of it. Rank is often gained or lost because you, dear player, hit yes on a bribe offer. As a new player, it may not mean much- particularly if you're not sure or don't care which kingdom you're in, or don't know any of your current lieges.
It happens. Lag, a misread line, "I thought I clicked no!" and other things. A lot of people decide- or are told- to not even click no on bribe offers, just in case a no click gets treated as a yes. Often the cause of a lot of excitement and drama.
You've been offered gold to change, and it's often enough to buy a few acres of land. Why not augment your income and switch? (More answers to that later...)
- Join Friends
You know the person sending you the bribe, and want to join them. The money is nice, but the chance to play with friends is even better. Sometimes, you may not have known them before, but they IM you and it sounds like a nice place to be, with people who are really interested in you.
- Leaving a bad situation
You don't like your current liege, or you feel forgotten. Perhaps other drama has taken place that makes you feel uncomfortable or unhappy where you're at. It's even possible that you've been told to "take the next bribe out". Hitting 'yes' on that bribe offer may be a relief, and a chance at a new career.
You know a fair amount, but want to see how it is done in other kingdoms. Perhaps you're an alt of a more experienced player who wants to keep his or her finger in other pies. This is a good opportunity to get into the inside of another kingdom, and perhaps gain access you wouldn't have otherwise.
Same old, same old. The kingdom doesn't change, the liege doesn't change, and nothing is happening. Perhaps you even quit playing for a while because it didn't seem worth your time. Then you see an interesting new name pop up on the hud, offering gold. Why not? It's at least something new...
And why wouldn't you want to do it? What are the downsides to taking that bribe?
- Burnt Bridges
Leaving someone behind can reduce their rank, certainly reduces their holdings, and if they're active at all (or if their lieges are active) is likely to cause hard feelings. The polls say that most people will take someone back if they're "genuinely sorry"... but those polls can't be fully trusted.
- Lack of subjects
People tend to drift away from someone who switches a lot. They liked it back in the kingdom you left- maybe they were friends there, or people who first joined in that kingdom, and aren't interested in changing. Or perhaps your now former lieges/friends approached them and asked them to come back, since you had left them. If a rolling stone gathers no moss, it's also true that a wandering TE players tends to gather no subjects. This may not be a huge factor for beginning players, but is more likely to be a problem later. You could end up as a Marquis or Duke with no one under you and no immediate prospects of anyone.
You decide to join a new kingdom that sounds inviting, and once you get there, you find that your new liege and his or her friends don't fully accept you. They know that people who come usually go just as quickly, and even after perhaps paying millions in gold to get you, they still put you "on trial" and don't really accept you, or treat you as "one of them". I once had someone tell me of an alt that they had used to explore other kingdoms with, using bribes. He said that out of about a dozen lieges, only two had even bothered to speak to him. It's a self-reinforcing cycle that often occurs- you're more likely to leave because no one trusts you, and no one is likely to trust you because you're more likely to leave.
So, there you have it from the viewpoint of someone being offered a bribe. Any other reasons to add, anyone?