From what I've observe, whenever a female is involved among males a fight is usually bound to happen. Certain actions the female do spark the aggressive nature in males especially when male and the female are in a relationship.
Usually this occurs when the male's level of interest is higher than that of the female. This could be the result of him getting sex after waiting for a very long time for it. He become entitled
This insecurity usually reveal itself when she become too friendly or the other male get too friendly.