When it was first reported that Adele was undergoing vocal cord surgery in October 2011, there was much speculation regarding what exact vocal cord pathology she suffered from (hemorrhage? polyp?) and what type of vocal cord surgery she underwent for correction (laser? cutting?).
To explain, a lesson in some basic anatomy first...
In Adele's case, she remembers the very moment this occurred during a radio interview when she "felt a pop" and her vocal pitch suddenly dropped into the bass range.
This makes perfect sense... To use the analogy of a violin string, the thicker the violin string the deeper the pitch. When hemorrhage occurs, the vocal cord becomes thicker due to blood pooling resulting in a deeper voice instantly.
Read more about vocal cord polyps here.
Of note, she later went on to sing a stellar performance at the Grammy Awards debuting her new voice for the first time after her surgery.