Well, for one, the immediate reason was Hitler's invasion of Poland. The general policy was appeasement was around even till 1939, however invading a country directly that the British said not to was the final straw. (Hitler's earlier incursions such as the Anschluss and occupation of the Czech was all seen as justified as a "German Reunion"). The stance against Hitler was still weak and unwilling and because of this Hitler took a risk and invaded. He had however not expected the British and French to retaliate. Even after the retaliation, the willingness to fight was low was we can see on French reluctance to invade into Germany even when Hitler's troops was tied up in Poland.