This has been happening the last 2 or 3 days now. Before the recent update, I thought this might fix it but no (I'd previously uninstalled the update as none of my apps were working).
So now it'll be on silent as I'd normally have it (I use my watch for notifications), then randomly it'll vibrate and be on vibrate, and autoroute! (I always have on portrait mode).
Any ideas? It's driving me mad! Have to keep my phone out my room at night so the vibrating doesn't wake me!