Automatic change of "initial size" of a database? I just had a problem : days ago, on one of my client's databases, the LDF file suddenly grew up to 100% of the disk capacity.

.TRN backups are correctly done every 15min (I don't think reducing this time will help, because this database is rarely used).

I created this database from a backup, two months ago. Everything was OK.

What I didn't understood, is why the "initial size" of the LDF file was... 512GB, which is the size of the disk...
If the initial size had been like that since the first restore, i would have noticed it. So I'm certain that it was not the case.

What I did : backup, simple mode, reducing initial size (it reduced the file size without shrink), full mode, backup. 

It's OK now, but i'd like to understand if the "initial size" can change automatically, and if yes, how to prevent it.

Thanks!
There is nothing that allows SQL Server to modify the initial size of a file automatically, by itself.

The file must have been altered by a sysadmin or someone with ownership of the database.

I expect they were setting the max size of the file and set the wrong option.