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? Or that Honda was annoyed he declined offers in the past? But I have never really seen any evidence of this so it was more just my gut feeling based on no facts at all other than that I couldn't think of a more conventional reason. I'm sure there was a good reason behind the decision ... just don't know what it was.
I guess this forum already discussed how odd honda's disapproval on Casey's request, with some additional information from people directly involved in motogp paddock. It all summed up in one word, "Alzamora".