The reason for sudden CPM drop may vary, but is often due to high traffic. When receiving a lot of traffic, the campaign will target both higher and lower bids, and the average will drop.
When sending less traffic the higher bids will be prioritized. So high traffic may lead to a bit lower CPM, but still higher total revenue.
Also CPM may seem lower than it actually is. Some kinds of popunder traffic has up to two days delay before showing in stats.
Keep in mind that your CPM depends on bids from advertisers, it is not set by Plugrush. The higher quality your traffic is, the higher advertisers will bid, and the higher your CPM will be. If your traffic doesn't convert well, advertisers will lower their bids to increase their profit.