Here’s my idea: when the site is inactive for more than a day or two, people who care about the community become a little apprehensive that it may be stagnating, and are a little more likely to post now rather than postpone their planned post to later. If the inactivity continues for another day or two, their apprehension will grow and may hit the threshold for several people on the same day, so you get an over-compensating spike.
Here’s my idea: when the site is inactive for more than a day or two, people who care about the community become a little apprehensive that it may be stagnating, and are a little more likely to post now rather than postpone their planned post to later. If the inactivity continues for another day or two, their apprehension will grow and may hit the threshold for several people on the same day, so you get an over-compensating spike.