|Image by Nat Ch Villa on Wikipedia|
Details are sparse in media reports, but given the voice rest order is for only a few weeks, I suspect it's due to laryngitis of some kind, whether bacterial or viral. If nodules, polyps, cyst, etc. were present, voice rest would have been for several months (rather than weeks) with possible surgical intervention. Regardless, voice rest is the normal typical requirement to recover from laryngitis quickly or after surgical intervention.
Steroids are often given as well to help accelerate recovery.
• Michael Buble - May 2016