One is thrown under the bus when they are made the scapegoat or blamed for something that wasn't their responsibility in the first place. A coverup for your mistake.
Physically throwing a person under a big, smelly city bus is the perfect metaphor for the act of positioning someone to be ground up under the wheels of the ever rolling omnibus of society in your stead. The bus carries people. The people's weight is what crushes the victim. This setting up of a patsy has an earlier, more agrarian beginning in "throwing him to the wolves." Someone is going down, it's not going to be you, so you select a candidate to feed into the system.
You get thrown under the bus when someone (usually a co-worker) reports some wrongdoing or slacking off to a superior or other influential person. Sometimes used with the suffix "Vrooooom!" to simulate the noise the bus would make as it passes by at a high rate of speed.
Example 1
I was feeling really upset with my colleague, so I threw her under a bus. This was my fulfilment of a lifetime urge to actually practice the phenomenon to "throw under the bus".
Example 2
The driver confronted me and asked "what the fuck did you throw her under my bus for?"