Getting caught in a thunderstorm is the most likely cause. This can happen even to experts if they are incautious enough to take risks with the weather.
Here's an example with a paraglider. (I'll see what I can find for a hang-glider)
Ewa Wisnierska was sucked into a powerful thunderstorm while training
for the world paragliding championships in Australia. She was carried,
unconscious and wearing a layer of ice, to an altitude of more than
32,000 feet into the eye of the storm. https://youtu.be/IXLdsnB5VBw
Here's another - this is a good one because it is videoed with a live commentary from the pilot who escaped the updraft before it was too late.
Caught in Cloud Suck! - Life lesson from 3Km above sea level
Hang-gliders are likely to find it easier to escape a storm because their forward speed can be much greater that that of a paraglider. They should be able fly out of it provided they can keep a sense of direction.
Hang-glider Beginner wing 14 mph to 45 mph. Advanced wing - 16 mph, to over 100 mph which has been clocked in speed gliding competitions
Paraglider Beginner wing 13mph to 22mph. Advanced wing - 14 mph to 35mph
EDIT (from the same source)
See under Wind penetration where my above suggestion is supported.
Here is a claim that it can happen to hang-gliders as well but there is no reference to back it up.
In one reported incident, two hang gliders were caught in cloud suck
by the same storm clouds. Both gliders lost consciousness above 30,000
feet, one, happily, was released by the storm, and regained lucidity
before crashing into the ground. The other glider was not so
fortunate. His frozen body was discovered later and returned to his
If you want to know how people get trapped and about emergency procedures for escape, then I suggest you Google "cloud suck".
Cloud suck is a phenomenon commonly known in paragliding, hang
gliding, and sailplane flying where pilots experience significant lift
due to a thermal under the base of cumulus clouds, especially towering
cumulus and cumulonimbus. The vertical extent of a cumulus cloud is a
good indicator of the strength of lift beneath it, and the potential
for cloud suck. Cloud suck most commonly occurs in low pressure
weather and in humid conditions.
Hypoxia due to Altitude
Helios Airways Flight 522 was a scheduled passenger flight from
Larnaca, Cyprus, to Athens, Greece, that crashed on 14 August 2005,
killing all 121 passengers and crew on board. A loss of cabin
pressurization incapacitated the crew ...
The Time of Useful Consciousness will vary depending on personal
physiological factors (e.g. if you're a smoker your blood doesn't
oxygenate as well - you will probably have less time. If you're a
mountain climber in excellent shape and used to breathing rarified air
on your climbs you'll probably have a little more time).