We used to have at least 3 ~ 4 highly populated servers, but now it's just pretty much one or, two during the weekends.
Like Umbra said, whichever has the highest Wheel of Money jackpot usually is filled with one or more players looking to win it.
This kickstarts more players coming in during the dead hours (everybody's sleepy sleepy time) as all other servers have less to no players. Most players then just pick whichever has the highest population.
During the absence of high jackpots (usually meaning those 1 million+ jackpots have been taken recently), Frankfurt 1 during EU peak hours or Dallas during US peak hours tend to get populated.