I think he thought that after pretty much deciding he was done and then being actually told by Mattiacci that he was done he thought any of the following would happen,
Honda would pay him huge $ to not go to Red Bull. Or better yet that Hamilton would have thrown all the toys out of his Mercedes pram and be going to Red Bull or Ferrari or back to Mclaren and that he could step into the Mercedes seat.
When Red Bull announced that A) Vettel was leaving and B) that he was joining Ferrari without Alonso apparently having sealed any particular deal and with Red Bull announcing Kvyat and also having told Mattiacci he was a son of a bitch it immediately reduced what Alonso can demand from Honda who can probably now just keep Button and Magnussen next season and sign Ricciardo or Hamilton for 2016 or force down Alonso's hand which had thought it had Honda paying him $50m a year (or driving the Merc).
Of course Hamilton may still throw his toys if he gets beaten by Rosberg although he does seem to have got the measure of him over the past few races. IF that happens Honda's position would be even stronger since they could play Alonso/Hamilton off against each other in contract negotiations.
I think it's very likely that he just won't be driving next year.
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