Brain Drain(Human Capital Flight).

Before the wall went up in Berlin many skilled laborers and professionals were defecting to the west as their skill would give them a good standard of living in the west. Once the Soviet Union collapsed this started again, and even moderately skilled workers would have a better standard of living in a economically stable region vs staying in places like Romania or Estonia.

Wikipedia is pretty good at answering this question, so you should either expand on which parts of that is unclear. For example http://en.wikipedia.org/wiki/Demographics_of_Estonia#Population gives reasons for migration with citations, so that's pretty thorough.