According to Lacrima, a "mago-historian" from Tir Tairngire, the eruption that formed Crater Lake occured on 3454 BC. This date has no real significance that I know of. That would be 341 years before the date given by Ehran for the end of the previous age of magic and the sinking of Atlantis (resulting from the explosion of the Theran orichalcuml pillars IIRC). However, the Tir Tairngire Peace Force cordonned off Crater Lake about one week after Lacrima released his findings.
Interestingly enough, in Real Life, geologists estimate the eruption of Mount Mazama that created Crater Lake happened around 5,677 BC ±150. If we assume the Fourth Age duration was the same as the Fifth Age, 5,124 years, this eruption occured really near he middle of the Fourth Age (the exact middle year would be 5'675 BC). That would be another assumption that the middle year is also the highest point in the Mana Cycle, when the Horrors would be at their strongest.
My guess is, whatever happened at Crater Lake in the past, the place is the reason the immortal elves who founded Tir Tairngire chose Oregon to do so in the first place, instead of any other region of the world where they could manipulate the government into welcoming metahumans.
The Tir elve calls the island in the middle of Crater Lake, "Tesetelinestea". According to Street Magic, this is Sperethiel that translates as "Skull of the Dragon." In his Will, Dunkelzahn gives the coordinates of dragon bones accros North America, a skeleton for which Dunkelzahn already possessed the head. So, maybe that head was at Crater Lake and Dunkelzahn retrieved it at some point, before the Tir elves set their shop there.
To push things even further, there is also the theory that the skeleton was Ghostwalker. Dunkelzahn wanted the skeleton to be on display at the Smithsonian so that Ghostwalker spirit would have a easier time finding it when he wuld exit the Astral Rift his brother was about to leave in Washington. Ghostwalker showed up at the same time as Shedim. That is, spirits with the ability to claim a dead body, and even regenerate it if they are powerful enough.
Also, Crater Lake started turning from a Domain into a Mana Ebb around the time of Dunkelzahn's death. Which is also the time the Astral Rift opened toward wherever Ghostwalker was. So maybe it was some sort air current flowing in your apartment, when a door was opened. What am I suggesting here is that Crater Lake was the place Ghostwalker first entered that wherever he was.
A simplier theory would just link Crater Lake shift to Dunkelzahn/Lethe destroying the Astral Bridge to stop the Horrors return.
