That was why may be rendered “that was the reason.”

Our fathers were handed over to the sword: Our fathers are the “ancestors” mentioned at the end of the previous verse. Were handed over to the sword (by God) means “God let their enemies kill them” (Good News Translation). For sword see 1.12. We may assume the defeat here refers to the actual defeat of Judah by the real Nebuchadnezzar in 587/586 B.C.

To be plundered may be rendered “to have their possessions taken by force” (see 7.26).

They suffered a great catastrophe before our enemies: “A great defeat” (Good News Translation) is a possible translation for a great catastrophe. Before our enemies means “at the hands of our enemies.” It is not the same idiom used in verse 15, where “in the presence/before the face of our enemies” is used.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Judith. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here.