Some of it is seasonal, for sure. But there's also a growing number of posters whose primary contributions are to things not related to fishing--or NFR/tangential stuff even in a FR thread. And yes, there are some people who do both. But there is an ever-growing contingency where the most important thing is to keep generating attention, likes, replies, follows--any sort of interaction, really. So whatever that takes to keep that going--and if fishing isn't that important in their life/anymore/never really was, then it's going to be mostly NFR content to generate it. This is only going to increase as the site becomes more and more popular and people become more and more accustomed to having their dopamine needs met online.
I don't check the main forum near as often anymore because so much of it is NFR and/or just not-meaningful content. The sub-forums maintain a degree of focus, but even those drift as they get more and more popular. I think this could be strongly influenced with moderation or policies, but it apparently isn't going to go that way. That's fine, it is what it is, and it's not my site to decide. But don't expect the NFR thing to get better. If you've been a part of other forum-driven sites that have grown considerably, you've seen this same pattern before.