Canonical and its stakeholders decided that a release candidate on 21 April, which is landing right before Easter, would be hugely useful for Ubuntu 11.04 that is set to be released just one week later.
What was decided upon was to instead put out a spin of Ubuntu 11.04 a week prior and to call it Ubuntu 11.04 Beta 2. Normally there is just one beta release in each Ubuntu development cycle, but because of this change of mind in scheduling, there will now be two.
Succeeding last week's Ubuntu 11.04 Alpha 2 build will be a third alpha at the beginning of March. The first beta is set to arrive on 31 March while this new beta two milestone is on 14 April. This 14 April date also marks their language freeze, kernel freeze, and non-language pack translation deadline.
The change in the Ubuntu 11.04 release schedule was mentioned by Kate on the ubuntu-devel mailing list. The final release date for Ubuntu 11.04, which is the 28th of April, is not impacted by this late change in the schedule.