RPM might be the good to monitor, but it'll never tell you why. CPC: (cost per click) and CTR (click through rate) as well as the number of impressions tell much more in my experience. RPM is essentially just the product of CPC and CTR - so you see the effects of either changing.
- Obviously number of impressions is something the reflect the popularity of said page(s), and it'll always have effect on it all.
- CTR: how interesting are the ads compared to your actual content compared. You can reverse this to how uninteresting is the actual content compared to the ads - but then you enter the slippery slope of MFA. If CTR changes without content changes, it usually means the ads changed - at least with similar visitor levels and similar visitor sources. Getting a peak of visitors due to media coverage usually also makes the CTR peak as these visitors are typically less interested and more mindlessly surfing the web when compared to your on-topic visitors and even more so your returning visitors.
- CPC: what's the value of an average click. It's again something you'll have little to no control over, -esp. due to the interest based junk- but seeing this go up might mean you've somehow managed to get better paying ads. Most of the time I see this it feels like there's one or more advertisers that have my site targeted in their advertising.
Aside of that there's numerous conspiracy stuff out there: Google to run some A/B testing, ... all of which are hard to confirm or even detect.