Well its usually cause as soon as somebody finds something that they dislike, or something about an mmo they believe should be changed... GW2 has already fixed it, as it takes practically everything shi/tty out of mmos. So hundreds of reasons.
Continents never before has became alive and there are so much you can do with them.
this idea goes with political system where king has to maintain their land from the dark side, or will be forced to resign.
there will be parts of continent being taking over by evil things every week and all the corrupted lands will change from green grass/nice weather to hail storm and broken landscape, dead grass or something.
in order to defend those lands, the king has to send troops to kill a given number of evil mobs on that land like BAMs to bring back its awesome looks and earn reputation,
but there can be weekends events where evil will come at a given time to try to take the lands back, where guilds of people will go and defend the land from incoming mobs for an hour or more forawesome rewards, reputation and good gears.
if fail to defend, the lands will be corrupted again and goes in a loop every week, and too much failness will lead to resigning that king and find a new one that can do the job
that event is fail because overcrowded people in one location, what i suggested was mutiple land in danger of invasion so we can spread out therefore no lag or low FPS issue
At first glance, it's a cool idea, albeit one we've heard a lot about recently. :P
What I mean is that it seems blatantly lifted from the dynamic events system that GW2 employs.
On another note, have you heard about the Nexus events Tera will be incorporating? Those might give you the dynamic "spice" you're craving.