Most of this is from MotoGP. This is an awesome montage' of motorcycle crash video clips. These crashes are from motorcycle races. The way they slide across the ground at high speed after the crash is wild!
The most common cause in the video is high-siding, where the rider gives too much throttle, the rear end loses grip & slides, the natural reaction is for the rider to ease off the throttle, the bike suddenly regaining grip has the effect of catapaulting the rider over the bike, hence 'high-side.
Conversely a 'low-side' is usually the front slides (but also the rear where it does not regain grip) & the bike goes down depositing the rider on the road on the 'low side' of the bike.
These are common problems in racing. On the road you should not be riding so near the limit for these to be a problem. To keep out of trouble on the road you must ride within your ability, the road conditions & traffic around you. If in doubt sign up for some training, it's better than learning from making mistakes on the road. Also wearing proper motorcycle clothing is a very good safeguard.