They mumble when they are not sure what they want to speak or they forget the word that they need to speak and they should think it over .
If they mumble it shows they are unsure of what they are saying or beaten down in their confidence onto which the forum they are given to utilize.
Not everyone mumbles but the answer to your question is that they are being lazy. To the contrary about why people stutter, well thats because they are trying to talk to fast.
Why does a person say mean things to someone and then apologizes about it later?