Can be caused by Radioactive Chain Decay, or the general detangling of atoms.
Notes on the nature of Extraplanar Detachment
For reasons not fully understood, different planes, or dimensions, can become entirely unreachable. This most frequently occurs with planes that are consistently travelled to. To date, there is no known correlation between ideological differences or similarities and chance of detachment. It is believed though that time travel within a plane can expedite detachment.
So far accurately predicting a detachment is impossible. A potential detachment can be foreseen, if proper records are kept, but the exact timing of the event is elusive. What's even more confusing is the occurrence of a near detachment. A plane may become unreachable for several decades then reemerge.
The most obvious indicator of a potential detachment is how accessible a plane is. Travelling to a plane requires exact runework. Over time the specifics may become inaccurate and runework needs to be modified. This is a natural phenomena called Planar Drift and will happen to all planes. If the rate of drift suddenly increases it can be indicative of an extraplanar detachment.
A plane can unexpectedly detach but with modern record keeping this is rare. Most planes will drift further and closer in waves. With each wave the drift becomes more extreme. The plane is considered detached when it drifts further to the point of inaccessibility, then never drifts back.