They were conducting a "census". The territories of English-speaking and Spanish-speaking congregations oftern overlap. So as not to annoy people any more than is necessary, the Spanish JWs typically don't offer a presentation if they encounter someone who speaks only English, since they figure an English JW will be there soon enough. And of course most English-speaking JWs can't make a presentation in Spanish, so there is minimal double-witnessing.
The Spanish folks have territory cards like everyone else, but it will list only the homes where Spanish-speakers are known to live. How do they know where they live? They take a "census" as described above. Whenever they encounter someone not previously marked down as speaking Spanish, they update the territory card.
In very transient areas, they may do a census every few years.