Jeff crashed the car because he got too paranoid about the cops’ presence and attempted to quickly flee the scene.
Overpowered by the paranoia and his nerves, he wasn’t able to drive properly and ultimately crashed into the back of a parked car despite there being a clear and easy route out of the situation.
Of course, there was always the chance Jeff may have crashed on purpose. His collision with the car was inevitably going to draw the cops away from the house and let Gene escape undetected. After all, the consequences of hitting a parked car while presumably sober won’t be as bad as being caught orchestrating a serious break-in and theft.
However, this is Jeff we’re talking about. He’s clumsy and selfish, with the crash being the result of sheer distress. He merely wanted to get away from the house and avoid being implicated in any crime in the event Gene was caught.