Delayed conference review notifications
From what you have written, it sounds like this is one of the many computer science conferences that is in the process of shifting to include an opportunity for author rebuttal.  

Conferences already often struggle to hit their intended deadlines for response, due to the challenges of wrangling reviewers---most do send responses either on their deadline or within a couple of days, but that often involves frantic activity behind the scenes on the part of the program chairs.  

Rebuttal rounds are amplifying this problem, as they are still new for many conferences and haven't yet been properly figured into their planning and scheduling heuristics.

Most likely, then, this is a case of conference organizers struggling with a new process and you happening to be the ones being affected by slow reviewers.

Finally, it is worth noting that they didn't miss the deadline that was promised on the website (March 31st), but only the unannounced "bonus" rebuttal deadline.

Bottom line: the particular situation you are describing is not normal, but also not an unusual failure mode, and an appropriate response is to ping the organizers after the deadline has passed by a few days.