Here's how I think is what happened / reasons why he left, might end up long post.

So imagine you're Yellow, its end of 2014 you'll end up being only member left in Fnatic while other members moved on. You probably don't have million dollar offers from China or even bigger offers from NA comparison what Fnatic offers you to rebuild a team.

You decide between retiring & rebuilding, you choose rebuild. You acquired 3 rookies & one average jungler, you work hard & hard work pays off because in the end you win spring split.

Before summer split starts your replace one of your players for greater potential, you work hard again & end of the split it paid off again, your rookies have grown to be world class players, you are world class team already, but you decide to bootcamp in Korea where you eventually become best performing team in practice (according to RO 85% winrate even against teams like KOO & SKT).

Worlds start, you finish top 4th, rich organisations signs two of players of your world class team, Fnatic made offers, but they have their vision how to lead their organisation, they wont pay up to same amounts of money what rich NA organisations pay.

Now you're in similar situation like year before, you know if you stay you can build up world class team again, but you also know that who is going to stop rich orgs buying your players again, you have to make decision.

You want to win worlds, if you stay with Fnatic, rebuild as strong team as last season, if you win Worlds with that roster, Chinese billionaires gets interest of your star players, in that case even rich NA orgs can't out bid them, but if you manage to win worlds, it doesn't matter because you can retire after you win, but if you finish top 4 again, you want to try another season to win worlds, but rich NA orgs will out bid Fnatic again & you have to rebuild if you want to try again.

If you decide to going NA, you'll make deal if you finish top 4 in worlds, no matter what you guys will pay anything to keep this same roster & we try again with this same roster, if you manage to win worlds, China steps in with their billions, but same case as in Fnatic, if you win you feel that you can retire, it doesn't matter if they outbid your players, because you're gone anyway.

Sorry for grammar errors, I didn't reread this to fix, remember this is just how I think what Yellow's thoughts were at the time.

And yeah, to note that Yellow did get best short & long term offers from Fnatic, Huni & RO however got big stacks that Fnatic didn't want to match / couldn't match.