I've run into something similar on a regular basis with my servers. Mine occurs on the full .bak as well as the .trn. I'm assuming SQL2K on Win2K(3).

If you have the the "Run integrity checks before backups" checked on in the maint plans, and the integrity checks fails -- usually because someone is in the database and it can't take it to single user mode -- it won't delete the backup files. You probably don't have this problem when you run the full backups, because that is at midnight. But when you are doing tran logs during the day it won't work and you see the tran logs build up.

The catch is that you have to completely rebuild all the the maint plans from the ground up. For some reason, if you just go in and uncheck the box it will still run the integrity checks before backups.

I built the job below to blow out all backups (trn and bak) between 2 and 7 days that are in the msdb log. I run it once a day and on error out of disk space. You should be able to modify it easily enough to just go after tran logs.

declare @file_name varchar(100)

declare FileList cursor for
    select  physical_device_name
    from msdb.dbo.BackupMediaFamily bmf 
    left join msdb.dbo.BackupSet bs on 
       bs.media_set_id = bmf.media_set_id
    WHERE datediff(hh,bs.backup_start_date,getdate()) > 48 
    and datediff(hh,bs.backup_start_date,getdate()) < 168 
    and physical_device_name not like '%BEXVDI%'
    ORDER BY bs.backup_start_date desc

OPEN FileList
FETCH NEXT FROM FileList INTO @file_name
 WHILE (@@fetch_status  -1) 
  BEGIN
    --print @file_name
    SELECT @file_name = 'DEL ' + @file_name
    EXEC XP_CMDSHELL @file_name
    FETCH NEXT FROM FileList INTO @file_name
  END
DEALLOCATE FileList

Jus my $0.02.