Solar flares

Your sun has quite powerful EM hickups. It is about to go supernova. It could be 60 years, or 60.000 years, they can't be accurately predicted. But there's a supernova coming soon, so any settling scenario would be short.

Geomagnetism

Your planet has tectonic activity and a layer of magnetic substrate in its crust. When an earthquake occurs, induction currents will cause EM pulses and your radio gets fried.

Unintended side effect of propulsion system experiments

Your settlers are hobbying with new spacecraft technology. They don't mind rules and regulations, they messed up their Alcubierre prototype. It causes EMP's now, these seem to occur in a wide range of space time everywhere around the planet and on various times. Every EMP event is an echo of the previous instance, causing damage in electronic equipment.

two natural, one artificial..

Solar EM flares

Your sun has quite powerful EM hickups. It is about to go supernova. It could be 60 years, or 60.000 years, they can't be accurately predicted. But there's a supernova coming soon, so any settling scenario would be short.

Geomagnetism

Your planet has tectonic activity and a layer of magnetic substrate in its crust. When an earthquake occurs, induction currents will cause EM pulses and your radio gets fried.

Unintended side effect of propulsion system experiments

Your settlers are hobbying with new spacecraft technology. They don't mind rules and regulations, so they messed up their Alcubierre prototype. It causes EMP's now, these seem to bounce around in space time, everywhere around the planet and on various points in time. Every EMP event is an echo of a previous instance, causing damage in electronic equipment. The echo of the event will slowly fade out in space time, eventually, but that will take 10000 years.