The continuing series of events that effect the outcome of your luck or fortune.
The phrase originates from the dependency on public transport, for instance how being late for the bus can trigger multiple events that messes up the whole day. However, this term is not exclusive to transport issues and can be used for any other circumstance.
Good bus physics is when everything runs smoothly and goes to plan.
"A woman cut in front of me in line earlier, which meant I was late for the train, which meant I was late for work, WHICH MEANT I GOT FIRE. That woman totally messed up my bus physics!"