In reply to Steph.

Another issue is improper singing. I worked with a voice coach for YEARS before I actually got a consistent grasp on how to properly place tones to avoid throat strain. I kept unconsciously falling into bad vocal habits. Along with breath support, tone placement to avoid vocal cord grinding is one of the other major tenets of proper singing. A person may sound good, but they may be singing poor technically.

And yet another factor could be fatigue. Justin Timberlake puts on a high energy show, and it can be really hard to attempt to sing and dance at the same time. When singers lose their breath, they stop singing and start croaking out notes. So, some of it is (in Ronnie Devoe voice) "placement."