It seems that in all cases where I have observed an LDE on the EME circuit (and in most of the cases that I've read about lunar LDEs where there was sufficient information) there was sunspot activity at the time associated with solar flares, aurora, radio blackouts, etc. Under these conditions it seems likely that the sun is ejecting large clouds of plasma. If one of these end up in the path of someone working EME, it's possible for the plasma cloud to cause a reflection as well as the moon. The ground station then hears both a lunar echo and an LDE (Long Delay Echo).

Depending on the size and position of the plasma cloud, one or both stations may detect it's presence. Because of the distances involved and depending on the density of the cloud only those station with sufficient antenna gain and Tx power may be able to detect these.

This is only ONE possible explanation. To hear a recording of an LDE on a NON man made signal check out the VLF Page !