In this video of a witness testimony at the George Zimmerman trial, the interpreter uses usted (as to be expected) until about 23:30, at which point he has a brief moment of clarification with the witness regarding pronouns in her testimony, and switches to addressing her with tú. He uses tú for the rest of the testimony.
What motivated him to do this? I am student interpreter and non-native speaker of Spanish, and am interested in the intricacies of formal and informal second person pronouns. Can any native Spanish speakers/interpreters shed some light on this situation?