what are the reasons of voltage fluctuations?

Answer 1

I assume that you are referring to house current?

Some fluctuations are caused by noise; some by other things which use electricity being turned on and off (kinda like the hot water in the shower vanishing and reappearing when someone runs water in the sink); some by problems with power lines.

There are more, but that covers most of the common ones.

Answer 2

Simply put:
The voltage at any point in the system is the result of
the resistance of the system to the current being drawn to that point.
The resistance loss is proportional to the current.
Large changes in the current drawn result in changes in voltage.

There are other reasons, but this is the most common, longest lasting, and easily observable.

Answer 3

“Overloaded, inadequate power generation and distribution systems, particularly when large step load changes are present, result in large voltage sags and surges. Voltage fluctuations between 155 to more than 310 V have been observed in the field.”

Answer 4

Reasons could be many
low voltage and high consumption ( disparoty in supply and demand)
transmission losses
short circuits in the supply lines
If its house hold connections unauthorised hook on
like welding machines for ex ; can cause bad fluctuations
like some one using a high current consuming motor
it could even be a bad wiring in your house!