To make generational GC efficient, you want to have very short lived objects, or objects that live forever. Lots of moderately long-lived objects is the worst case scenario, as it causes permanent fragmentation of the old GC generation.

Lucene generally churns through a lot of strings while indexing, but it also builds a lot of caches that live on-heap for a few minutes. Because the minor GCs come fast and furious due to indexing, that means you have caches that last just long enough to get evicted into the old generation, only to become useless shortly thereafter.

The end result looks like a slow burning memory leak. I've seen the worst cases take down servers every hour or two, but this can accumulate over time on a slower fuse as well.