I'm betting an in principle deal was established with Brisbane to trade Yeo for 28 assuming other conditions were met, one of those being West Coast's ability to obtain a 2nd round draft pick. As such, pick 31 was irrelevant to the deal. That's one potential dealing that I think is quite likely. .
So why did it take so long for the deal to go through? We got pick 31 days ago. Brisbane wanting to announce everything at once in the hope it hurt less (a.k.a the bandaid theory?)