Time machine is designed to run at low priority in the background. It works differently than most other backups.
A "complete backup" for a given date&time consists of all the unique files, plus links to all the older files in older backups that are already saved in that Backup set. It only adds the changed files to the large body of files it already has on the backup drive, and makes "hard links" to take advantage of the old ones already present on the backup set.
Since it is intended to work in the background at low priority, it computes 'what files need to be added to the backup set' FIRST. Then it makes certain there is enough space on the backup drive and consolidates older backups until there is space, and only THEN does it begin to add files to the backup set.
New backups rely on, but generally do not disturb, older backups, unless the drive is too full for the additions.
But as I mentioned above, the computation to determine 'what needs to be backed up' requires it to read the directory entries for every file, and may take all afternoon if you have not done a backup in the last few days. This could fail if the source drive directory is scrambled, but is unlikely to cause any disturbance or corruption in your old Time Machine backups.