Full Idea
Epicurus said that not every proposition is either true or false. ...Epicurus was afraid that if he admits that every proposition is true or false he will also have to admit that all events are caused by fate (if they are so from all eternity).
Gist of Idea
Epicurus rejected excluded middle, because accepting it for events is fatalistic
Source
report of Epicurus (fragments/reports [c.289 BCE]) by M. Tullius Cicero - On Fate ('De fato') 10.21
Book Reference
Cicero: 'On Fate, Stoic Paradoxes, Oratory', ed/tr. Rackham,H. [Harvard Loeb 1942], p.217
A Reaction
Epicurus proposed his 'swerve' in the movements of atoms to avoid this fatalism. Epicurus is agreeing with Aristotle, who did not accept excluded middle for a future contingent sea-fight.