Galera has two types of on-disk files to manage write-sets, a ring buffer file, and an on-demand page store. The ring-buffer size is controlled by gcache.size, but when a transaction write-set is large enough to exceed the size of the ring-buffer page ('gcache.page_size = 128M;'), then independent pages are allocated to cache write-sets.
An unusually large accumulation of on-demand page files can be caused by a combination of large transactions running on the cluster due to a data import or batch job at the time of an SST or if joiners are slow to process transactions.
What can happen then is the disk fills up with gcache pages and SST hangs indefinitely until manual intervention happens.
One thing that can save the disk from filling up in this case is to have a new variable that creates a global limit on size that can be written.