During the time of the Babylonian siege, the king and princes of Jerusalem decided to free all the slaves in a sweeping manner in order to alleviate the situation of the city under siege. However, as soon as the Babylonian army left Jerusalem and the siege was lifted, the motive for liberating the slaves was canceled, and the people therefore restored the slaves to their previous status.
As opposed to the breach of the covenant in chapter 11 in which Yirmiyahu describes the violation of many many mitzvot, here only one mitzva is violated. Why were the people punished so severely for the violation of a single commandment?