Malbim: Before the Bris, many poor people were slaves to their creditors. At the time of the siege, Tzidkiyah feared lest they rebel, or go out to Nebuchadnetzar to uproot their bondage. Therefore, he made a Bris to free them. So happened to the Romans in ancient times. At the time of war they needed to free those enslaved to them due to their debts, to quiet rebellion. After the siege ceased, they returned and seized [the borrowers] for slaves. They transgressed (a) the Bris and Shevu'ah, and (b) seizing them for slaves after they were freed is kidnapping.