I'm almost positive it has to do with how much money moves through your account (total bet, not won/lost) whether it is from higher stakes poker or the 888 casino.
I don't play high stakes poker, I was invited to 888 Elite after an unusual couple of months.
I had a $10 casino bonus from a club scratch card and decided to try my luck with the slot races and had the golden horseshoe as I racked up several top placings winning a couple of $5 bonuses, a $20 bonus and I finished 2nd in a really big one and got a $400 bonus.
I actually cleared almost $170 from the $20 bonus as I got the $400 bonus right after that and made some big bets that were credited to the $20 bonus since it came first.
I played that $400 bonus a long time and had it up over $1200 (nowhere near clearing it) but I figure my total wagers with all those bonuses was probably $15000-20000 (not won or lost, just total bet including all wins and losses), before I finally lost that bonus.
A few weeks later I built up from $100 to over $1200 real money over a couple of weeks and again probably had total wagers of at least $10000 to win that (I did quit and cash that one out).
So, over that relatively short period I probably had $30000+ in total amount wagered which maybe made me look like a "whale"
It wasn't long after that I got invited and have been reaping the benefits ever since.