Quite a number of satellites that can be observed are tumbling, and this causes their brightness to vary with time. This variation can often be seen with the naked eye, and the frequency of the brightness peaks can range between several per second, to once every minute or so. A good example is the recently launched SL-16 rocket, which is currently rotating at 2 or 3 times a second. The tumbling can be started by several mechanisms, but the most common is probably out-gasing. This happens often to spent rockets which have some fuel left over in their tanks once the engine shuts down. Sometimes this fuel is vented deliberately by ground controllers to reduce the danger of an explosion which would create many pieces of orbital debris and pose a hazard to other satellites. Other times the fuels leaks out gradually.
Occasionally, objects will explode in orbit, and the force of the explosion will start the resulting pieces spinning.