Yes, they need to stop letting people behind him.
I was left with the impression that the attack came deliberately from behind Trump. Trump’s first reaction was a slight crouch— as if he had suddenly been jostled (eg while riding on a subway car) from directly behind him. Then he looked down, but not focused on anyone in particular, more of a surprised look on his face and momentary confusion as to what he should do next. Then one guy jumped up to protect him, followed by several others. As soon as they determined the threat was gone, most of them immediately jumped off stage. At that time, one guy (not the first guy) remained on stage. Trump looked at him and got some kind of non-verbal signal that everything was OK to continue. Then that final guy jumped off stage. After that, Trump continued with his speech.
I got the impression that security might be more tight in front of Trump than behind him. However, someone else with more knowledge might be able to clarify if this is actually the case or not. If it is the case, then the protestor may have taken deliberate advantage of the weakness in Trump’s protection entourage (and that would be a bad thing). Trump’s protection entourage might benefit from analyzing the incident with particular emphasis on strengthening protection against threats from the rear.
(As if anti-Trump protestors were intent on demonstrating their cowardness by attacking Trump from behind, like a coward would.)