Sudden raise in size of incremental cumulative backup

Hi,

I have a customer who is using RMAN incremental backups, he runs level 0 backups once a week and incremental cumulative in between. Due to a failure his level 0 backup didn't ran the 20.3, in the following week he had a 30% load increase, but it's still a very moderate load, 200 logfiles of 256M in one week. As you can see below the cumulative backup is growing at a moderate rate until it suddenly goes from 75G to 1.2 TB

the number of blocks changed in vbackup_datafile is large, but I still see no logic in this sudden raise. Have any of you seen something similar