Buck Converter Voltage Fluctuation
That would be because the control loop can't follow fast enough, however this doesn't happen only in buck converters, it happens in all power supplies.

A solution to that is to put a big enough decoupling capacitor at the output, that can catch that transient without causing a significant drop, giving time to the control loop to respond. Also, if you have a long transmission line from the supply to the load, you may need to add a decoupling capacitor close to the load too, as the line inductance will also generate a voltage drop on the load.

Also, if the control loop of the supply is not properly compensated, you can get ringing at the output.