How to troubleshoot large transaction log file size?
It's probable that you had a long running query that caused a bunch of transactions. Something like a large batch of inserts/deletes/updates. You can track these down in the query store if you have a recent version of SQL Server, or you can query the plan cache for long running transactions. 

Once you've found the query, you can figure out what it's doing and go from there. You may just be stuck with it, but finding out what's causing the large amount of transactions is your first step.

Also, you should be aware that an autogrowth of 5MB is extremely small, and can result in a lot of Virtual Log Files when it grows, which can hinder startup performance. I suggest looking into tuning the growth rate as well.

https://www.brentozar.com/blitz/high-virtual-log-file-vlf-count/