It's usually due to a corrupted file. Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, delete or rename session.json, restart TB a few times and see if tabs open at exit are open at next restart.

I had this problem too as I had search results showing in tabs that disappeared on exit. I eventually realised that these aren't saved, it is only open email tabs that are saved. If session.json is examined with a plain text editor you can see what tabs are in there - only those containing open emails.