I have this same problem as well. It is very frustrating. I had to spend a ton of time running this down. The problem manifested itself by multiple instances of Word opening on reboot. Very frustrating. After searching for an answer, I thought I had found it in a potential add-in problem. But then by testing add-ins, I noticed that multiple add-ins caused winword.exe not to terminate on closing Word.
So more searching brought me to this thread. I tested having all add-ins enabled and disabling winword.exe as a protected application. Immediate fix! So the problem is definitely Malwarebytes