In this video, CBS sports announcer and former NBA player, Clark Kellogg interviews President Barack Obama and Prime Minister David Cameron at University of Dayton just prior to the Western Kentucky vs. Mississippi Valley State match up in the first round of March Madness (Western Kentucky won, 59-58). Notice how many times Kellogg puts his left hand on The President's right shoulder. It's rare that you'll see this repeated behavior directed towards a president or past president (or any other high ranking public office holder, CEO, celebrity, etc.) outside the confines of a close personal friendship and even then - not in a televised or public setting. In fact, the President himself reciprocates this touch - but only once - to Kellogg and then to Cameron as well. Kellogg knows Obama personally though, and what we're witnessing here is a subconscious bonding-rapport behavior - albeit excessive. Violations of personal space can quickly destroy rapport.
Another common body language explanation for excessive violation of personal space - is a dominance display, common with but not limited to that of narcissism/narcissistic personality disorder.
Not surprisingly, interpersonal space intrusions can also be a signal of affection and physical-romantic attraction.
It is important to exclude medical causes when interpersonal space is chronically violated. Damage to a portion of the brain (amygdala), borderline personality disorder, sensory processing disorder and various other neuro-psycho-social disorders are all important considerations.