After observing a migration of skilled workers to North America from post-war Europe in the 1960s, the scientists at The Royal Society coined this term.
•UNESCO reports that 10% of the 300,000 skilled workers in the African diaspora have PhDs
•Less than 50% of the 80,000 registered Nigerian doctors practice in the country
A number of studies have been done to try and determine the cause of brain drain, and many fingers have been pointed at pursuing overseas education as a major contributor.
Iran: According to the IMF 150000 educated Iranians leave the country yearly.
China: Stanford reports that since 1978, only 275000 out of 1060000 students returned home
While there is a majority of overseas students who choose to emigrate to the country they've studied in, this doesn't establish a cause-effect relationship.
So while it may not be fair to assert that overseas education is an outright cause of brain drain, it does seem to act like a catalyst for this occurrence.