I think (knowing or unknowingly) the parent is neglecting the child's emotional needs and development. Kids learn to manage their emotions really quickly if they're listened to and get to understand what makes them sad, angry, frightened, happy etc.
If the parent can't relate to the kids emotions and just checks out the kid doesn't necessarily get to know their own emotions, unless they're getting enough feedback from someone else.
Even then, it's scary to have a parent that relates in this way. You don't know what mood they're in, and can't feel secure in their company, so there's issues with attachment there if not really deep emotional wounding.