0.15.1 was released just shy of 5 months before 0.16.0 was released. I wouldn't say that is "too many releases in a short time", after all we do releases every 6 months (or we try to).

Anyways, the point of releasing 0.15.1 was to get a release which had additional handling for forks in order to avoid network problems with the 2x fork that was planned to happen at that time but got cancelled. It had additional handling for invalid blocks and such to avoid troubles with forks in general. That is why that was released. We considered it an urgent release because of the 2x fork which could disrupt the network.