Why is my transaction file growing when db is in simple mode and there are no waiting transactions
Your file is probably large but empty.

In SSMS, right-click on your database and go into Tasks, Shrink Files. DO NOT HIT OK IN THIS DIALOG - but it's the easiest way of seeing how empty your files are. Choose Log in the dropdown, and I imagine you'll see that your log file is actually 99% empty. 

You probably had a large process that grew the file to 160GB. You should probably shrink your file and then resize it to 160GB again manually (to reduce fragmentation), and then monitor how full it is using PerfMon counters.