From TF2 Wiki
Knock back is a side effect of some weapons that pushes a player away from the source of the knock back. In Team Fortress 2, knock back varies from very slight (only preventing you from moving forward temporarily), to extreme (launching you several feet into the air). Knock back can be useful strategically, such as when juggling opponents.
Knock back can also be used to interrupt taunting animations (especially taunt kills) and item uses (knocking back a scout before he finishes drinking a Bonk! will prematurely stop him from drinking).
Sources of Knock Back
There are several sources of knock back in the game ranging from weapons and buildings, to the environment itself.
|Weapons with knock back||Severity of knock back|
|Rocket launcher rockets||Causes players to be launched through the air.|
|Sticky bombs||Causes players to be launched through the air.|
|Grenade launcher grenades||Causes players to be launched through the air.|
|Scattergun fire||Pushes players back a bit if the shot was fired point blank.|
|Force-A-Nature fire||Causes enemies to be launched through the air, launches an airborne wielder in the opposite direction.|
|Sentry gun automatic fire||Pushes players away from the sentry, can pin airborne targets to the skybox or send them flying at high speeds.|
|Sentry gun rockets||Causes players to be knocked backwards a few feet.|
|Compression blast ability||Causes players to be launched aerially backwards for several feet.|
|Sniper Rifle fire||Pushes players back a bit if they're in the air.|
|Huntsman arrow||Pushes players back a bit if they're in the air.|
|Non-lethal Melee weapon critical hits||Launches players aerially if they're in the air.|
|Pumpkin bombs||Pushes players back. Sends them flying if they are caught in the explosion while airborne.|
|Natascha||Focus fire pushes players back at point blank.|