Transaction log grows day+1 2-3 hours after long running transactions

We use MS SQL 2008 R2 with Idrea SqSafe for Backup 

we have setup 

1. Full Databae backup once a week

2. Diff databae backup every day at 10pm

3. Hourly trans log backup 

 

the database server hosts 400 databases.  There were two incident we saw 

1. When we large updates to couple database. Engeineers shrunk the database trans log (using SQL managemnt studio UI) after the large updates compete on each databae. Let's say 2am.  The engineers confirmed there are plenty of spaces ( over 200 GB)

2. Next day 24 hours later after the large updates.  The drive where stores the Safe backfile reported low free disk space and the disk drive for trans log files reported low free disk space too. 

Query the safe backup records. It is obviously the trans log backup file grew 24-hours later  and continued for 2 hour.  For example, day+1  3 am grew 3GB, day+1 4am grew another 5GB.. then day +1 5am the trans backup file size was back to normal .. like 100MB. It happened to not only a databae but several database even the databae were not updated.

 

Is this a known issue ?   How we troubleshot this issue ?  It seems to be alright most of times, but whenever there was long running updates.. 24 hrs later, we saw low free disk space. 

Any suggestions or recommdation is appreciated.