Imperator is not synonymous of emperor, although is the root of it. Through the history of Rome, the title evolved to the meaning of a general especially able who had won an enormous victory. Traditionally, it was the troops in the field that proclaimed a man imperator, an acclamation necessary for a general to apply the senate for a triumph.
Since a triumph is the ambition of every Roman politician, history is full of cases were legions are bribed to call someone imperator.
See also: imperium