Might one reason be recurring famine?:
Despite its name, Cape Verde is an arid landmass with minimal agricultural potential. The excess mortality associated with its major famines in unparalleled in relative terms. A famine in 1773-76 is said to have removed 44 percent of the population; a second in 1830-33 is claimed to have killed 42 percent of the population of seventy thousand or so; and a third in 1854-56 to have killed 25 percent. In 1860 the population was ninety thousand; 40 percent of Cape Verdeans were reported to have died of famine in 1863-67. Despite a population loss of thirty thousand, the population was put at eighty thousand in 1870. Twentieth-century famines in Cape Verde were less deadly, but still extreme relative to most contemporaneous ones elsewhere: 15 percent of the population (or twenty thousand) in 1900-1903; 16 percent (twenty-five thousand) in 1920-22; 15 percent (twenty thousand) in 1940-43; and 18 percent (thirty thousand) in 1946-48…
…such death tolls imply extraordinary noncrisis population growth. For instance, if the population estimates for 1830 and 1860 are credited, making good the damage inflicted by the famine of 1830-33 would have required an annual population growth rate of about 4 percent between 1833 and 1860 — despite the loss of a quarter or so of the population in 1854-56.
Here are the author's working papers on famine.