Ive always wondered about that, Honda wanted him to ride for them on previous occasions but he declined. I don't really know why he changes his mind to step in for Dani. Possibly from a combination of all the pressure in the past and feeling obligated in some way to help the team.
What I found even stranger was Honda who had asked him on occasions in the past and then decline when he offered up. I kind of thought it might have had something to do with possibly upsetting their lead rider MM, upsetting the apple cart or ruffling feathers so to speak. But I have never really seen any evidence of this so it was more just my gut feeling based on no facts at all. I'm sure there was a good reason behind the decision ... just don't know what it was.