While that's true, I'd argue it doesn't usually equate to a major increase in long term activity.
To answer the OP's question, 'What's the secret of these small starter niche forums for getting so many daily posts?', the answer in the majority of cases is they serve a purpose. That purpose might be to provide unique content or simply an engaging environment that people want to join.
If your forum serves a purpose then everything else is secondary e.g. some of the largest most active forums run on free or ancient software using the default theme.