My son was like that when he was 19 and home from school. I never figured out why but by the time he was 21 he was pleasant again. Maybe it's a developmental stage - the terrible late teens?

When we went through that with our son, my husband and I decided that kids who've been away from home just hate coming home to be treated like a child again. Working on that theory, we decided to stop acting like parents to our "grown up" son and we stopped telling him what to do, stopped asking if he was alright, stopped being concerned about his responsibilities or his activities. We treated him more like a peer that we LIKED. When this happened, he stopped mumbling and started acting normal again. I don't know if this is a cure-all but it might not hurt to try this approach.