Reading the thread, I've seen several possible factors mentioned that can explain why several major franchises saw important iteration-to-iteration debut week's decreases: sequel fatigue, a way too crowded period for releases that increases the risks coming from competition, possibly customers getting more cautious after several titles released with major problems right at launch, or represented delusions compared to what massive advertising / marketing campaigns sold to them, alternatives getting stronger and stronger as their reach keeps on growing.
However, I'd say that there's another factor that I've not seen mentioned enough, and that IMHO is even more relevant in UK, compared to other countries: I'm referring to games getting relevant price cuts not that far from launch. The UK retail market is particularly competitive, so much that games tend to get price cuts / soft bundled with hardware faster than in other major markets, from what I've seen by reading PAL Charts threads in the recent years. Especially if the games in question fail to sell as expected. Now, this retail stores behaviour has possibly increased its strength, if we take as examples several insane deals featuring both PS4 and, especially, Xbox One: bundles with 3+ games and great prices, both during Holidays and in normal sales periods. I firmily believe that such strong policies favouring stock movement, due to their significant presence and influence on the market, are starting to influence the customers themselves more than expected. Basically, they're starting to wait for deals more and more because they've seen so many times titles getting important price-cuts even a few months after the original release, which translates into them wondering "What's the point in getting this title at full price when I can buy it cheaper just by waiting a few weeks / months?" And it's not like there aren't alternatives: online-focused games and extra-gaming alternatives fit perfectly in this scenario as distractions from other titles.
I see this as something similar (the market behaviour in itself, not the mere execution) as what happened and keeps on happening in the mobile market: years and years of F2P apps dominating the market have taught to customers willing to see what's new on mobile storefronts that the value of gaming on there is very low; thus, their spending-ceiling-per-title is extremely low, and even titles at $4.99 can encounter major difficulties at getting decent ROIs (there are some exceptions, even among indie developers, but this is the specific trend of the market for now). In the same way, while retail gaming customers are far from being that picky, have started to reconsider their own spending-ceiling-per-title because the market itself is teaching them that, most of the times, it's not worhty to get games at launch at full price, when they'll surely get important price drops not that far into the future.
Of course, if customers themselves don't see the titles as "exciting" enough to feel they need to get them as they launch, this kind of behaviour can only get stronger. Which brings back the "sequel syndrome" mentioned several times in the thread.