Background
Mere months ago, Yzek was a relatively accomplished small time smuggler. Working in tandem with his partner, Bam'fadu, Hes enjoyed a surprisingly steady income for such a job. It was his compulsion that drove the group apart. That and the Imperial fighter pilots who did that literally.
Yzek and his partner would prefer to not put the ship with the valuable cargo through customs, and instead Bam'fadu would go through, with a land vehicle. Hes would then fly towards a predetermined location, trying to evade patrols, and drop the load without landing. The Twi'lek would then deliver the goods.
After a particularly terrible chain of Yzek's bad bets, they would become desperate for a quick job. Desperate enough to run through a more heavily guarded planet than they were used to.
They began their routine as usual, but Yzek would get noticed by a patrol a lot earlier than he would like. He had to bail as the reinforcements came to intercept him. Then, an Alliance fleet arrived. His pursuers were fought off temporarily, but the Chadra-Fan was taken into custody before he could finish his part of the job. The Rebels fled, successful at their raid.
Yzek was now left without money, without job, and without a partner. It was only sensible that he would accept a grudging invitation into the Rebel forces.
Motivation
Crime.
Yzek has little interest in the rebellion, but is forced to work for the Alliance at this point.