You are confusing "rebels" and "Rebel Alliance". The first is people rebelling, the second is an alliance of groups of the first. The Rebel Alliance as instituted by Mon Mothma and Bail Organa was started roughly 2BBY. However, there were rebel cells all over the galaxy since the formation of the Empire (to be pedantic, they have been around a lot longer).
The TV show Star Wars Rebels follows the actions of a single rebel cell on the planet of Lothal. For the vast majority of the show, this rebel cell is only concerned about Lothal, its not until the very end that they join up with the galactic wide Rebel Alliance. Saw Gerrera is another example. As seen in Rogue One he ran his own rebel cell that was not affiliated with the Rebel Alliance.
So to answer your question in the title, the Rebel Alliance was founded around 2BBY (we don't have an exact date yet in current canon). However, "the rebellion" you reference in the body of your question is almost certainly a preexisting rebel cell that, pending survival, would have likely eventually joined the Rebel Alliance 8 years later.