There are 2 things needed to revert a civilization back to the stone age, loss of equipment and loss of knowledge.
Loss of equipment:
An EMP won't be enough as mechanical devices are unaffected by it (water wheel, gun, old non electronic car engines) so it has to be something that actually destroy all of the equipment around all the civilization, chances for that happening naturally without it also killing everyone is slim to non so most likely it was planned somehow, could be a religious as the colony was overtaken by a cult convinced that all off world tech is evil or could be just a mad scientist created a gray goo nanobot to eat everything man made.
Loss of knowledge
Even if you all of your colonists woke up in the morning to find out all their stuff is gone (don't ask me how their bed vanished without them waking up) they are colonists, they must be used to hardship and have tech knowledge enough to build some tools from what's available in the woods, I'm positive that out of 20,000 at least one person knows some bushcraft which already puts him ahead of anything that can be considered as stone age, I've seen bushcraft buildings being created within a couple of weeks that included rain collectors and fireplaces (with chimneys) made without any real tools and only from materials gathered, that means that people knowladge also has to go, either you mind wipe them to forget everything (and I mean everything, if even one person remember a book he read in the past then he remembers what writing is which is already ahead of stone age and can be simply replicated with a pointy stick on a piece of wood) or you kill off everyone with any form of knowledge leaving only the babies around to start from scratch, you will have to find some way to care for them without them learning anything more advanced then stone age tech in the process so maybe AI robots designed to look like caveman that are also programed to hide the fact they are robots from the babies until they grow to a point where they can take care of themselves at which point the robot goes to "hunt" and never return?
Bottom line is there is no realistic way of having an advanced society return to be stone age without the society (and I mean everyone in that society) choosing to revet to that stage, we simply learned too much since then and even a tiny bit of that knowledge remaining will put us ahead of stone age tech.