They have found a table that the players are loose. They are hoping to get a crack at them. That is the way the pros play they look for weaker players. They call it no foldem holdem players !! Free money.
To add to that, you can click on the actual tables and see where the players are from. If they`re not from England then they're not playing on Zyngapoker, and if they are from England it's at most a 50 50 chance that they`re on Zyngapoker.
Let's keep it realistic chad the most of any of the tables that have waits are 3-4. There are a also lot of tables that aren't full or empty as well (of the same limits and table size) waiting lines exist on poker tables if people see a reason that they really want to sit at that specific table, for example if the average pot size is significantly higher than the other tables people will want to sit there because that usually implies that there are a few loose players at the table, there are other reasons as well, like if people see players they know or like playing against they might join the waiting line.