Sometimes lifters get injured not from lifting weights, but from preparing to lift them. Getting dumbbells into position can cause shoulder problems once the load gets heavy. This is especially true for the dumbbell bench press.
Most people lean all the way back, get the weights close to their chests, and then lift up from there. That can cause shoulder irritation. It's not the best way to do it.
Think: Rock, lean back, kick. Your shoulders will thank you.