MariaDB uses all /tmp space
SHOW VARIABLES LIKE 'tmpdir'; -- You could change that (and restart) to immediately workaround the problem.  But that is not a long-term solution.

Complex queries with inadequate indexes are likely to be the cause.  Set long_query_time=1 and turn on the slowlog; wait a day; run pt-query-digest or mysqldumpslow -s t to see the 'worst' queries.  Focus on them.

If you don't see how to improve a query, post a question with EXPLAIN SELECT..., SHOW CREATE TABLE and a tag it [mysql] [query-optimization].