One theory (not my own, someone else spelled it out to me) is that both members of this couple were facing a change in lifestyle, whether they wished it or not - post-Brexit.
No longer could they spend all the time they wished touring Europe. In fact, both France and Spain would soon be limiting them, legally, to 90 days every 180 days. So they would have had to return to UK every 90 days, and stay for 90 days, before being legally allowed back in the EU.
Nothing has been said in MSM about them applying for dual citizenship, probably because they didn't meet the qualifications (one needs to show employment in the new nation, etc).
Anyway, someone I work with put forward the theory that DC may have been preparing himself mentally to go back to UK and "settle down" while ED wanted to continue their van lifestyle. To do this, however, she would have to get very used to a new lifestyle wherein she was not in compliance with visa regulations. People apparently do plan to do this, there are even online forums about how to "live underground in Europe" in the new Brexit world.
I'm not sure how I feel about this theory, but at one level, it does make sense that if Esther was planning to be more adventurous than ever and disagreed with DC about going back to UK, that she'd leave the dogs (and even the van) to continue wandering. However, personally, I think this would denote some deeper unhappiness and issues with regular life. In my professional life, I've observed, met and interviewed several such people (longterm residents on national lands in the US, for example, who are not exactly within the rules of the national parks). This kind of very solitary life appeals to some people (John Muir is an historic example).
The fact that the dossier doesn't address this new citizenship issue is interesting to me and I'll watch with interest to see if we learn what DC (and the dogs) do next. Perhaps DC was in fact employed by someone to housesit for a long enough period of time that he could have applied for some kind of settled status - in France.