I think Google tests how pages perform at different spots. One of the ecommerce sites I manage, the product pages sometimes start doing wild bouncing around across the top 50 spots, before settling at a particular spot - high or low - and just sticking there. I always figured Google was testing how that page performs at different rankings, and what the user behavior is, and then settles it out accordingly.
That doesn't necessarily mean it's connected to a manual review. Sometimes a coincidence *is* just a coincidence - even if it happens twice.