Balloons - when they are closed up and pumping hot air into the envelope, very little escapes apart from residual, radiated heat. Hardly a killer.
When the envelope deflects and there is a large spill of hot air, or when the contained gas is vented to stabilise/stop the balloon from lifting, then a bubble, a shaft of air gets released. Being HOT in relation to the surrounding environment it is going to travel skyward with the characteristics of a scolded cat. If it meets a flexible nylon wing in passing what will that aggressive burp of energy do?
I don't suggest that this happened to our unfortunate fellow paramoteur - but it might be another good reason to avoid le ballon methinks.
New sport, new interactions with associated lessons to be learned? This poor guy may have stumbled into a situation that will be recognised in future - he may have saved lives or injury by exposing a hazard to us all very graphically.
The AAIB will hopefully have something interesting to say about it later in the day.