They call this the rolling shutter effect.
Also known as the jello or wobble effect and is caused by the side to side motion of cameras that have the CMOS sensor.
The rolling shutter effect can be caused by:
fast moving objects,
moving or panning the camera quickly, or
camera vibrations.
Why does this happen?
The CMOS sensor on the camera reads information a line at a time, from top to bottom. Thus different parts of the image is recorded at DIFFERENT times, thus creating the wobble effect.
Cameras with the CCD sensor do NOT have this problem. This sensor records the whole image at once. Thus avoiding the rolling shutter effect.
There are pros and cons for using different image sensors, however it looks like we are moving to a CMOS world.
This site below explains the differences between the sensors very nicely if you are curious.
http://www.bhphotovideo.com/c/find/newsLetter/Comparing-Image-Sensors.jsp
REALLY interesting effects you can do with Rolling shutter effect.
For example, the water looks like its static. PetaPixel explains it; "by pressing a water tube against a speaker, bibio was able to control the vibration frequency of the water flowing through the tube. He then adjusted the pulses of the water to match up with the frame rate of his Canon 5D Mark II."
Probably the one example seen the most is the helicopter with static blades flying around the airfield. Just had to share.
Hope that helps