inhospitable logs in SE

I've noticed that running Informix SE (5.0) on an SS10, that from time to time the permissions on the database logfile are changed from 660 to 640, and thus all db users get an error -224. I've seen it often, and so the fix (chmod) is automatic, but I can't seem to figure out what would cause this. The database is only accessed by various esql programs, and then almost always sequentially. Any ideas? Thanks, Jason