It happens a lot, titles are vacated rather than fight mandatories. Warren obviously has a lot of leverage with the WBO and chose fights which would progress his fighters to no.1, he can't really have foreseen that Collins and Nelson would retire.

Hennesey seems to prefer the WBC route, and Witter and Froch (if he wins) will win vacated titles.

Cotto, Margarito, Jones Jr, Tarver, Hopkins, JMM, Judah are just some high profile boxers who also had first titles which were vacant. (OK, I went on Boxrec and of about 20 or so bigish names I searched just under half were in this vacated group)