Rationale
Sakai has historically followed a strategy of 'code freeze' in anticipation of a release, whereby the entire trunk is branched at a particular snapshot in time, and from that point forward only verified bugfixes are merged into the branch.
This has been effective in allowing development work to continue while still closely guarding the stability of the code being tested for release. It has also had the benefit of simplicity: a single deadline for everyone to get their work done in time.
Particular components of Sakai are often stewarded by particular teams, evolving on their own timeframes