"Seeding" is about playing on a server so that it will eventually become (and sustain being) full.
"Seeding" rarely happens on unpopular servers, since it is hard to sustain that population.
So, the seeding button shows servers which are likely to become sustainably populated.
The most useful way of seeing this is through the population graphs in the Skial stats, we can see the history.
So, if you look into the stats pages for the servers, for instance:
http://stats.skial.com/#server/105
Red shows when the server is empty, green dots show when it is full.
We can see three peaks where there are green dots, so this server is filling up.
The peaks are going towards the green dot, so we see it will probably become full soon.
This means if you were to join now, you would get more credits from the seeding bonus,
Since it shows that a server is going to be successfully seeded - compared to trying to seed a completely empty server.
The reason why it tends to be Payload, I believe - purely speculation:
But I believe Payload is probably the most boring gamemode to play by yourself - so it empties out quickly.
Compare this to our most popular servers, 2Fort EU+/LA+, they rarely empty out.
EU+ hasn't emptied out completely this week at all (lowest being 1 player)
LA+ last emptied out nearly a week ago.
The graph is from EU+, it doesn't need as much help filling up, as it is often full.