Children are heartless beasts at times. My daughter has gone through a lot of 'I don't like Daddy' or 'I like Mummy best' phases with absolutely no provocation (she occasionally does the I like Daddy best but usually I've EARNED that through thwarting her in some way). I always tell her off for it but she doesn't seem to get that it's hurtful to him. (it's also deeply uncomfortable for me! I don't want her to feel that way)

Considering how often children change their mind about which flavour of icecream they like best, I don't think it means anything, it's just them working through ideas in their heads. Also, as I said, HEARTLESS BEASTS.