These have electrolytic capacitors in them, that tend to charge up all by themselves due to their chemistry. A high voltage one that's been discharged can reach 30 volts or so. (Kilovolt caps need safety shorting straps if you are working inside equipment with them, eg high power radio transmitters.)
When the volts get high enough the circuit will burst into life momentarily, discharging the capacitor, and start over again...
