Could be many reasons, who knows. Maybe they were looking at something on their phone that evoked their reaction, maybe they heard something in their earphones, maybe they suddenly saw a bird nearby that scared them, or maybe they just remembered something which ended up causing it.
The latter happens to me occasionally, and I am fairly certain it has nothing to do with Tourette's. Usually it is me walking somewhere, then remembering something I forgot that i needed to do or having some realization, and I just end up going "oh fuck" or "ooooh". Not loudly like a yell, but definitely audible to someone within a 5-10 feet range.
It could also be what you are suspecting as well, but without actually talking to the person, there is quite literally no way to know. So imo it is a rather pointless exercise.