Heavy, awkward and frequent lifting are all to blame for back injuries from time to time. How heavy is too heavy? Everyone will have his own limit. Know yours, reduce the weight of the load (through repackaging or the use a lifting aid) or prepare to spend a lot of time convalescing on your back.
Awkward objects -- those that are bulky, above your head or below your knees -- can cause us to break from good posture in an effort to work with the object's bulk, odd shape or distance from us. When lifting, be sure to keep your core muscles engaged, try not to lean forward while moving the object and never extend your arms while reaching or carrying (which can increase your risk of not only back injury, but also injury to your shoulders, neck, wrists and hands).
And, finally, frequent lifting adds to your risk of injury because it can cause repetitive stress and muscle fatigue.