First of all, these variations do not look important, also it depends if we are talking of thousands of PV, tens of thousands, hundred of thousands etc...
But reasons can be multiple :
- brand safety - evolution of the profile of your visitors (also more page view, doesn't meant "quality" extra visitors) - evolution of mobile usage (compare desktop vs desktop , mobile vs mobile) - evolution of the advertisers, and the demand for your site - continuous increase of ad blindness - ...
Speaking very generally, there's not as much demand for your traffic.
Noone can tell you exactly why're you experiencing a drop, but justpassing has already given quite a few possible indications. The market is in a state of constant flux; when your average RPM decreases, that of other publishers might increase (possibly at your expense) or remain stable. All you can do is work towards becoming as attractive as possible to advertisers.
Nope, but the number of advertisers doesn't necessarily correlate with revenue anyway. Everything roughly boils down to your attractiveness to advertisers, especially compared to the countless other available websites they can advertise on. If they want (or the system deems it effective) to be seen next to your content, or by your visitors specifically, that will get you results. If not, the result may be a downward slope.
I think it's important for AdSense publishers to know how AdWords works. Ad buyers can indeed choose specific sites to advertise on, but mostly it's the system determining where the best bang may be had for the buck. It's most certainly not random. As a publisher, you can optionally define ad placements [support.google.com] that advertisers can target.
It's the start of a new financial quarter, and regardless of what people say, Google DOES shave, to claw back money on any drop from June-July-August, or previous years. It's happened consistently for years. Then - things randomly improve after around 2-3 weeks, once Google have clawed/shaved enough off Adsense users.