Many theories are being thrown around, but the trend in the headline does not seem suspicious or surprising at all even though it's a country often associated with fraud. I grew up in Nigeria and personally know plenty of Nigerians who regularly travel back and forth between London and U.S. cities where Uber operates, plus it's the most populous country in Africa so if even a tiny fraction of the country is traveling abroad, that would be a large number of people and probably a large percentage of downloads in a category (Travel) which the rest of the population doesn't use as much.