The rate of aftershocks in the sequence initiated by the DPRK underground tests has been increasing since January 2021. In total, 22 reliable aftershocks were detected between January 13 and October 1, 2021. Their characteristics are similar to the aftershocks in one of two clusters: 1) the fifth DPRK (DPRK5) test (mb(IDC)=5.09) conducted on September 9, 2016, which induced the first DPRK aftershock in the sequence detected at 1:50:48 UTC on September 11, 2016; 2) the sixth DPRK (DPRK6) explosion (mb(IDC)=6.07), which generates its aftershock sequence with characteristics significantly different from the aftershocks in the DPRK5 sequence. The length, intensity, and alternating character of these sequences suggest specific mechanisms of energy release likely associated with the interaction of the damaged zones of the DPRK5 and DPRK6 and the collapse of their cavities with progressive propagation of the collapsing chimneys to the free surface. According to the depth estimates based on the moment tensor modelling, the DPRK5 and DPRK6 were conducted at practically the same depths. The difference in magnitudes suggests that their damaged zones differ by a factor of 2 or more. The first aftershock of the DPRK6 (mb(IDC=4.12) 8.5 minutes after the test is evidence of the cavity collapse and creation of a chimney, which did not reach the surface. The activity in 2021 indicates that the chimney collapse is not finished yet. One can expect more aftershocks in the near future, likely ended with the chimney reaching the free surface.