There may be a number of causes, but a major one is weakness in the diaphragm, the large dome-shaped muscle under the lungs.

Most people (non-singers, non-actors) tend to breathe shallowly. Good "diaphragmatic breathing" feels like you're breathing with your abdomen. The diaphragm moves down when you inhale, pushing your abdominal organs downward and outward, back up and in when you exhale. [Watch the abdomen of a sleeping dog. They good use diaphragmatic breathing naturally.]

Voice students hear a lot about the diaphragm from their voice teachers, who give them exercises to strengthen and control that particular muscle. That's the key to that all-important issue of "breath control" for singers of any genre, opera, pop, OR folk.

Breath control is also important for actors, particularly stage actors (you want your lines to be heard beyond the third row without sounding like you're shouting).

Russian bass Mark Reisen's voice was still strong and without a quaver at the age of ninety.

Don Firth