The phrase "unexpected resignation" means that someone has decided to quit their job, but their decision was surprising or not anticipated by others. Full definition
With Duncan's resignation coming as a surprise — and only one week after an equally unexpected resignation from Speaker of the United States House of Representatives John Boehner (R - OH)-- the education community is left questioning the future of Congress's year - long efforts to replace NCLB. (ascd.org)
By the time that election comes, six of the nine members of the court will have been appointed by the current prime minister (and maybe more — there's always the occasional unexpected resignation before retirement age). (canadianlawyermag.com)
The third to watch out for is the unexpected resignation of staff from sensitive position. (profitableventure.com)