I would not totally reject the theory about the seasons,it definitely affects people in many ways.Let's say that the winter is maybe time when people spend a lot of time in the indoors,and accordingly to that spend more time on computers/internet-but in the winter in the northern hemisphere we see that it is always the toughest part of the year for cryptocurrency.So it might be connected with Christmas-New Year holidays when people typically spend most of money in the year,and sell crypto for fiat.

On the other side at the end of the summer and early autumn people start to buy BTC just because have somewhere in the subconscious they could profit in few months and have more money for the holidays.Between all this is the summer and long-awaited annual vacations,time in the year when most people leave BTC for some time.

I can tell for myself that I'm much more active online in the winter,but how come the warmer weather I spend more and more time outdoors and I assume it is similar to most people(at least for those who have seasons).