SBI, as a server, surely didn’t decide to tank, but in the week (maybe two… but I don’t recall) before league play, I can say that one of the largest WvW guilds on SBI was almost completely absent, and this was intentional according to a friend of mine in this guild. When league play started they turned out in force. IMNSHO, this guild’s absence made the difference between SBI being in Silver instead of Gold.
I hope Anet addresses this problem by making the reward for the last place in Gold significantly better than first place in Silver, and similarly for Silver and Bronze. Then next time maybe we won’t have to deal with such nonsense.