What are some of the reasons for voltage spikes in voltage regulated DC circuits?
The chips may consume current in pulses (think a clock or microcontroller running at 1 MHz).

When connecting such spikey current consumer ICs with long wires (on PCB) to voltage regulator, the stray inductance of these wires cause the voltage to drop momentarily at the chip during a current pulse. Which is why you need decoupling capacitors to keep the impedance low at the chip.