InterSystems FAQ rubric
If a relationship is set and there is a large number of n in a 1:n ratio, a large amount of memory may be consumed due to sequential processing of the relationship.
After referencing a many-sided object in a program and internally swizzling it, simply releasing the variable containing the OREF (deleting it, setting another value, etc.) will not free the many-sided object and the relationship object. This is the cause.