In the last week I have had this three times. I get all set up and my bike is at the Just Ride point (I guess that’s what it is called :)). I am doing something like getting my music ready, switching the TV to the right channel, etc. What I am not doing is peddling.
Then I look up and my bike has, um, started without me. I’m toodling along at 17mph and my pedals are still 
This can go on for 30 seconds or so.
I am a bit of a stickler for comparing my own times/distances so I have to start Zwift again (which takes a bit of time) - there’s no way to reset while staying in the app, as far as I can tell.
More annoying than anything else but I thought I’d report it.
I have a Kickr and, apart from the occasional gap where I randomly lose ANT+ (probably 3-4 times, totaling a minute, in almost 700 miles of riding, so pretty reliable generally) I have had no issues.