Mostly this will be a change of the log4j configuration (file appender).
Thanks for the response. We did not make any changes to the log4j recently. At least not for past 8 months. Please find our log4j configuration attached.
Without seeing what's in the "huge log file", and how it is different from a regular log file, it is impossible to tell why there was a difference. Wolf-Dieter's guess about log config file changes is probably the best guess. It has happened more that once that someone changed a config file while the app server was running, and the change didn't get registered until the app server was restarted.