All of this was probably done on Romney’s advice and direction. Dealing with Romney is always career ending.
How quickly we forget that this incident came out of the disgraceful Cruz/Rubio tag-team against Trump in that debate. Rubio was vilified, and Cruz walked away unscathed. I figure it's because it will be easier to take Cruz down later in the game.
Rubio was doing quite well when he was being respectful of Trump.