Since most nicknames come from silly things soldiers do... Let my imagination run away a little here:
Mission Log: Planet P3X-537
Incident report: Weapon discharge
According to her on-site report, the soldier in question 'saw something running towards me' and fired her weapon without verifying her target. It is worth noting that this was her first combat mission and the mistake was understandable, though still not acceptable. The creature killed resembled a large insectoid approximately four feet in length and two in height. This burst of weapons-fire alerted the enemy to her squad's location. The operation was barely salvaged but successful nonetheless. It is my suggestion that command be lenient when determining what disciplinary action is appropriate.
Note: Her squad has started calling her 'Bug Killer'. I don't think they are going to let her forget this one any time soon.