Men not wanting to be responsible for their part in making a woman pregnant see violence as a way of getting out of that responsibility. In civilian life, they tend to just run away, whatever that may mean. In the military, running away is a jail-able offense, making violence an even more likely option.

It is even more likely if the relationship was unlawful in the first place. In the military there are rules on who can be in a relationship with who, depending on rank primarily. Officers cannot have relations with enlisted and non-commissioned officer (sergeants and above) cannot have relations with lower enlisted. This is because the military understands the unfair power dynamics created by a rank based hierarchy (even if sometimes they chose to ignore these rules for higher ranking soldiers).

A pregnant soldier is extremely vulnerable to domestic violence and it keeps a lot of women from wanting to join the military, and understandably so.