That's the thing... There is a "DocumentDate" field, but it's not selected as segment. There's no other date field defined.
I've been thinking about it on and off for hours, and the only thing I can think of is, at load time, it's placed in the cache with an expiry date of today+90 days... Then the cache expiration runs weekly, and the eligible files expire. Either that, or maybe it's using a field from the arsload table. I'll have to dig into it some more when I'm back on site.
-JD.