I think it happened because of the fact that you use BTRFS filesystem. And how you can to resolve this problem:
1. Login as root to Konsole
2. Perform: snapper list
(this command list all of your existing snapshots and it's numbers)
2. To delete several snapshots simultaneously perform:
snapper delete n-n
(where n is a number of snapshot, you want to remove (the order is: snapper delete n_from-n_to). Don't forget if you delete the pre snapshot, you must to delete its post snapshot.
3. After few minutes perform:
(to see how much space was freed on the root partition). For more information, i recommend you to read Opensuse Snapper article (google it).