Analyzing what is causing the log file to grow

For one of our database we have an issue where its log file got increased rapidly last week on Fri and Sat.

The database is on SQL  server 2008 R2 with compatibility level at 80.

Please see below log grow events :

First, we thought Index maintenance like Re-index and update stats could have been the reason, but when check the shceule that job ran on 16th using below code:

USE ABC
GO
EXEC sp_MSforeachtable @command1="print '?' DBCC DBREINDEX ('?', ' ', 80)"
GO
EXEC sp_updatestats
GO

I know above is OLD fashioned, but we believe that should not be the major cause here?

How can i determine what happened on 14th and 15th which cause the event to trigger and log file bumps to 80 and 70 GB both days.

Kindly suggest, thanks