Grizzly loves to get his exercise during cage cleaning time. I'm not sure why. He seems very happy about the cleaning, always popcorning. But maybe he also figures Bean is also occupied by the cleaning, so it's a good chance to dash about without bothering him.
The funny thing is, if he can't find a good loop to run while I'm cleaning the front of the cage, he won't hesitate to rearrange things to create one.