I had the good fortune to watch Milton's last shed...

mid-shed pic

In this case, at least, the poo happened about 30 mins after the actual shed. I think the shedball is caused by them doing the actual shedding in their hides, so they have to pass over themselves so much it wads up the shed.

As for why they poo, they have to use all their muscles to get that skin off- if I ever get that video resized, you can see how they have to undulate to get out of it! I think it just stimulates their bowels in the process.