The only time I've seen ARSMAINT expiration suddenly running a long time is when a migration from another platform to CMOD was performed some years ago (say 7 for example).
At that time a lot of docs were all set to expire on the same date.
In 7 years' time everyone forgot (or was no longer there) that there was a big expiration coming up.
Suddenly, tons of expiration processing.
Oh no! Is expiration processing running wild?
Are things getting expired that aren't supposed to be expired?
No, it was an unexpected increase in volume.
This is what we call the "pig in a python" problem.
Ed