not a good title I know. But just look at top month user profile score diagram ,More of them are descending. So where are they? I truly afraid about all negative points to this question, But I can take lots of example.
So you're talking about this chart... This is mine.
This isn't a running total of reputation, it's a daily chart. It's how much rep a user earns per day. Some days aren't as good as others frankly because we can't all have 700-rep days every day. There aren't always perfect/popular enough questions or we just have other stuff to do.
And given that it's just turning into summer, a lot of people do have other things to do and would much rather be out on a bike than melting in front of a computer.
I wouldn't worry about it. I don't think it's a trend.