Ramp Agent reported a tow bar malfunctioned during push back of an A321.

Narrative

I began my push out of [the gate]; the safety pin was in. The shear bolt was in place. The Captain told me to leave the aircraft abeam the gate. With that instruction in mind I went out a little wide so I could back up nice and straight and land perfectly on the line in front of the gate. As I began to go in reverse I felt a slight bump and looked at the tow bar. Nothing appeared to shift and the bar was nice and straight. The plane began to shift on its own which is when I told the pilot to apply the parking brake. As I stopped; I saw the plane going in a different direction so I immediately told the Captain to apply the brakes. I'm not sure why it occurred when the bypass pin was in and I purposely pushed the aircraft out wide to help prevent from having a broken tow bar. After the plane stopped we called the Crew Chief over to take over the situation and lead us as to what to do next. I'm really not sure what I did wrong. As I can recall I did everything right. This seems to me it was just a freak accident.