Database not naturally shrinking log file I have a database that is set to Simple recovery mode. There is 99% free space in the log file and I can manually truncate the log file right down to 500 MB.

The next day, after a daily import process runs, the log file will be back up to ~70GB.  

If I check log_resuse_wait in sys.databases it returns 0 which would indicate nothing is blocking a shrink operation.

Why isn't the log file naturally shrinking? Also, how can I find what SQL queries are causing the log file to grow to such a size?
You must check log_reuse_wait during the import process. Most likely you have a some long running transaction started at the beginning of the import that is pinning the log for the entire import duration. Simple Recovery model log is automatically truncated, not shrunk. See How to Shrink the SQL Server log.