If this article is to be believed:

A series of appearances by similar formations were extensively studied at Toppenish Ridge on the Yakima Reservation in Washington in the late 1970s by Greg Long. The lights at Yakima were videotaped and triangulated by several teams organized by Long, including participation by the U.S. Forest Service. The lights were discovered to be emerging and following some of Toppenish Ridge’s fault lines—nearly 100 of which are easily visible. After the eruption of Mt. St. Helens in 1980, the Yakima lights all but disappeared. This is an interesting clue to the phenomenon.

• article by Greg Little