Fritz got separated from his squad, and shot several times, and had to stagger to the only shelter he could find, only to die begging for his mother. This scene drives home the point that every soldier on the battlefield has a family to go home to, a life outside of this crazy war.
For everyone other than Welkin, Alicia, Largo, Rosie and Zaka, death on the field is permanent. Add this to the fact that every recruit has their own model, personality, biography, and fully voiced death sequence, and this trope is a possible reaction to JUST ABOUT ANY SQUAD MEMBER dying in battle. Listed below are some particular sad deaths:
SusieEvans: "This is why I...detest war... Too many have...already killed and...been killed..."
CatherineO'Hara: "Heh heh...It's strange...I feel this is...better than losing...a comrade..."
Lynn: "Ahh... Karl... I love you... I'll always..."
Ted Ustinov: "I-Is this... really the punch line...? If so, I...I'm not laughing..."
Aisha Neumann: "Welkin? Help, I'm scared... Please don't let me... I don't wanna die..."
The Darcsen concentration camp being burned down at the end of mission 10 is a disturbing enough scene on its own. Taken in context with the later revelation that Rosie's family died in a fire caused during a Darcsen hunt makes her reaction all the more potent. Seemed that one hit too close to home.
The Imperial soldier's death in Chapter 8 also qualifies. It's not too difficult to see how hearing him cry out for his mother can trigger Vietnam flashbacks for some people...
Once you've played through Selvaria's DLC, you'll never feel the same way about shooting Oswald the Iron dead in Chapter 10.
Welkin crying in Isara's room after they retake Bruhl.
Hell, everything from Isara's death to that point there is a major league tearer.
Isara's and Welkin's last moments together, Squad 7 mourning for their comrades who just died and Welkin crying over failing to protect Isara.