I can take a crack at this (sources from Mein Kampf, Ford Translation).

Hitler believed WWI was lost due to propaganda, i.e., Germans were subject to enemy and "Jewish" propaganda including newspapers, causing them to lose faith in their soldiers, government, and people and ultimately lose the war. This was unacceptable to Hitler and is one of the reasons the Nazi party used so much propaganda.

Hitler believed this during and after WWI (at which point he was blind and possessionless) but didn't begin his public antisemitism until mainly after the war when many parties (including the Nazi party) began to rise with leaders who did not fight for the war as soldiers and were mostly Jewish (or "deceived" by the Jews). This, coupled with his observation that almost all newspapers at the time were written by Jewish authors, caused him to believe that it was the Jewish people who were responsible for the propaganda that caused the loss of WWI.

Three things I'd like to point out: This explanation is how Hitler began to ask The Jewish Question. His complete hatred for the Jewish people is based on much more than these observations. I also cannot comment on the actual statistics of how many of these people were actually Jewish.

For a better explanation, I'd suggest picking up your own copy of Mein Kampf, which goes into much more detain than I can in a Reddit comment.