Say that the bus carries a single bit a certain value, to read this value, you're going to connect the input of another circuit to the bus via a switch.
Let's assume that the value on the bus is one and has a known capacitance associated with it. And also assume that your input circuit has a known parasitic cap at its output and its initially discharged.
The instance you're going to connect the two caps together, charge from the bus will go to the input parasitic cap, until both caps have the same potential. If you apply the law of charge conservation, you can predict how much drop to expect on the data bus.