When landing an elytra at high speeds (say, after using a few fireworks), momentum still carries over. After landing completely, activating the elytra by jumping will launch you at a high velocity the direction you are facing. 

It is a little hard to explain, so I have attached a video of the issue.

Ideally, after you land at a high speed, all momentum should be lost, and you shouldn't be launched when just jumping.

In the video attached, this bug appears twice, each time I land.