Hi,
I had the same problem as you and was very disappointed. Up until this problem I was very happy with the QC35, which I have used for hundreds of hours.
I followed the below YouTube video a few weeks ago, and the problem has not returned.
https://www.youtube.com/watch?v=r3qHSaWi4X8
The process took about 20-30 minutes and was relativity easy, however some parts are quiet small and it took a few tries to get them into the correct position.
The problem is the switch becomes loose and touches multiple contacts, making the headphones turn back on. Don't bother with all the other troubleshooting advice Bose has told people to do (updates, resets, pair & unpair etc), the problem is a physical problem with the switch.