Have you ever made something yourself. Have you spent the time to do it right and when it was done you thought it was perfect? Well, kittens tend to think their poops are just like that. They bury them to save them for posterity.. so when you go digging for them, they are very interested in why your interested in their creation. They have to watch you dig for them, watch you find them, then watch you deposit their precious cargo in the bag. Often they have to poke their little heads in the bag to see if your actually snagging their creation.
And if your truly lucky.. they'll turn around and make you a new one after you've collected all the ones they have made already.