In short, Rome was a republic, ruled by two elected consuls and the Senate. Caesar (who was elected consul multiple times - already illegal) seized power using his battle hardened legions (who were very loyal to him after multiple campaigns in Gaul) and became Dictator for Life. Since he was a king/emperor in all but name, the Senate feared he would crown himself king and destroy the Republic.

Also, 'et tu Brute?' was invented by Shakespeare for his play, just wanted to point this out since it's a fun fact that not many people know.