A Lifeline player gets launched in the air after an explosion lifts her off her feet. The scenario couldn't have been more perfect. Caustic was dealing with an enemy Lifeline before he eventually fell.
Lifeline came to finish the job, but instead of simply shooting the unarmed Caustic, she went to execute him. As the execution screen took place, a grenade landed beneath her feet.
Right before the Lifeline goes to finish off the execution, the grenade detonates, launching her in the air and saving the Caustic. Most of the replies in the subreddit were of players who have seen the same thing. Sometimes grenades land perfectly beneath a player, and you'll be launched in the air. The height of the launch depends on the health of the player. If a player dies like this Lifeline, the player should drop faster since they've been knocked. If you still have life, the game doesn't try to get you to the ground, so you'll go farther.
Either way, nothing is more satisfying than an enemy player being knocked moments before an execution finishes.