Brahmin tipping is an action possible in Fallout 2, Fallout 3, Fallout: New Vegas, and, at one part, Fallout Tactics. This is an Easter egg, likely a reference to a quote from Tandi in Fallout, where she describes what there is to do in Shady Sands. She says that "brahmin-tipping gets old real quick."
Tipping can occasionally be achieved by selecting the "push" command (the palm icon) in the drop-down menu that appears when a person or item is click-held, however this will also sometimes simply prompt the brahmin to move away. Performing this action inflicts no damage to the brahmin.
Fallout 3 and Fallout: New Vegas
By crouching unarmed near a brahmin and pressing the activate button you will push the brahmin over, causing it to go into a ragdoll state and getting back on its feet in a couple of seconds. You will not lose any Karma for doing this, nor will anyone or anything become hostile. However, repeating this action may cause the brahmin to die. Even after it dies, no one becomes hostile and you do not lose Karma. You may loot the resulting corpse for brahmin steak. In New Vegas, each time you tip one, it counts towards the "Knocked Up" challenge.
Brahmin tipping features as a special encounter in Fallout Tactics. During it the player will find several brahmin surrounded by a fence. By interacting with the brahmin, the player can tip them over.
Behind the scenes
This action is a reference to the real-life urban legend of cow tipping. In the real world, cows do not actually sleep standing up, and are actually easily disturbed. Depending on the complacency of the cow, attempting to tip one would take between 2 and 4 people to actually succeed.