There is a new feature in Marklogic 11 that relates to overflowing to disk in order to protect memory. It is only listed as an Optic feature. However, since optic is SPARQL under the hood, maybe the feature is kicking in and using disk.

The link below describes the feature and also various ways to see if it is being used.

https://docs.progress.com/bundle/marklogic-server-whats-new-11/page/topics/new-features-in-marklogic-11/adaptive-memory-algorithms-for-optic-sorts-and-joins.html