For some reason, maybe to protect himself against sensory overload or so, Sherlock retreats to his mindpalace. He stays close enough to the surface that he can follow John's simple cues, but he does need some extra prompting and doesn't pay attention to his surroundings other than the bare necessities.
+ Sherlock didn't explain in advance. Maybe John realises something's wrong when Sherlock suddenly isn't with him anymore and he has to double back to find his friend right where he left him.