Seems like that change in attitude first showed up in the little shack in the forest, with Tohsaka making them strip down.

Maybe it's the whole "shared magic circuit" connection, or the fact that saber's been essentially reduced from an insanely powerful servant to a nearly helpless little girl (thanks to mana depletion from the fight with Rider, and the overwhelming strength of Berserker). Maybe Shiro's self-sacrificing "serve the servant" behavior is just really starting to get to her?