Why does the voltage drop on a bus with the increase in load?

The source of power is an ideal voltage source having zero internal impedance with a series impedance that can have both real (resistive) and imaginary (reactive) components. This impedance is for example the transformer impedance plus the impedance of the feeder to the bus. This is called the Thevenin's equivalent circuit. This is the bus voltage. When you connect a load to the bus you are completing the circuit with another impedance. So you have a circuit with a voltage source and two impedances in series. When you draw more loads you are lowering the load impedance. This increases the voltage drop across the source impedance. For example if the load impedance were the same as the source impedance the bus voltage would drop to 50% of the open circuit voltage. If it was zero impedance, the load would be a dead short and the bus voltage would be zero. This is how you calculate the short circuit available current, from the open circuit voltage and the internal impedance. The overcurrent protective device on the output of the source should be rated to handle at least the short circuit current.

According to a footnote in NEC which refers to ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning) the voltage at the load should be no less than 95% of the open circuit voltage at the building service entrance, that is no more than a 5% voltage drop. This is to assure feeders are sized sufficiently to avoid excess I squared R heat loss in the wires.

For more details you need to refer to a power system analysis textbook or Google. If the power flow equations are analyzed carefully, especially the Newton-Rasphon method, then you'll be able to understand and see the close relationship of reactive power and voltage (the change in reactive power with respect to the change in voltage). As a side note you'll be also able to analyze the relationship of real power and the voltage bus angles. This is the basis that leads into decoupled load flow or DC load flow, cutting down computation power and time.