Thanks to your post, I learned something.

An excerpt from heavens-above.com:

"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...The tumbling can be started by several mechanisms, but the most common is probably out-gasing."

The full text is here.