alci As far as I can tell, there's not something specific that triggers a release. A release is pushed when it needs to be released. If there's a significant change, like a Vanadium update, or an urgent patch, or a new feature, a new release will likely pushed ASAP.
As for your second question, from the documentation:
Releases are tested by the developers and are then pushed out via the Alpha channel. The release is then pushed out via the Beta channel shortly afterwards. Finally, the release is then pushed out via the Stable channel after being tested by some users using the Beta channel. In some cases, problems are caught during Beta channel testing and a new release is made via the Beta channel to replace the aborted one. In general, it's not possible to downgrade unless a downgrade update package is generated, so use the Stable channel if you cannot tolerate dealing with temporary issues while a new release for the Beta channel is being created.
More info at: https://grapheneos.org/releases#about-the-releases