Your average military dictatorship is able to maintain power, over long periods of time and despite great unpopularity, through simple brute force. Thus, if it's to be brought down at all, it often must be brought down from within. Sometimes, such an internal coup is staged by a small faction of young, reform-minded, liberal officers who have become uncomfortable with the harsh, typically right-wing policies of their elders and superiors. This we call a "young officers coup".
The "young officers" are in a unique position of advantage: they have both (a) physical power, as army officers in command of troops, and (b) massive popular support, as reformists who are toppling a hated authoritarian regime. Also, one could say that (c) the idealism and freshness of youth, is a third factor in favor of these junior conspirators.
A famous example of a Y.O.C. is the 1974 overthrow of the Caetano military dictatorship of Portugal - as soon as the young officers took control, they wrote a new constitution, insitituted democracy, and released Portugal's foreign colonies from imperial bondage (most notably Mozambique and Angola). A much less successful example of a Y.O.C. can be found in El Salvador, where a group of young officers seized control in 1979 but were unable to consolidate power and prevent the country from spiralling into a hellish civil war.