I know I have very close to full coverage (I am 9 glyphs from having them all). I know not all my competition has every one of them, but there are enough players that everything is covered by several posters. With me posting as hard as I am, and them posting as hard as they are, how can glyphs randomly post at fall back prices with that much coverage?
This is a side effect of QA and heavy undercutting with long posting periods. As the price gets lower and lower, eventually it reaches the point where only one person is willing to post at that price. To QA, this glyph is now under the threshold for everyone else. QA will now skip this glyphs until the price goes back up. Either the low priced ones get purchased or they expire. If they expire, every glyph that was above that one will expire first.
This glyph becomes that last one. It holds everyone out of posting that glyphs until it disappears. Once it is gone from the AH, the next person is open to post at whatever price they want. Someone who works hard to reset the price of glyphs will see this loner and reset the market. But those people don't take the time at every posting to do that.
The colors on the left indicate the value of the glyph. Red is near cost, orange is minimal profit, and green is all the mark up. Everyone undercuts in the green area, few will do so in the orange. This will cause many glyphs to cycle in price over and over. The worse the glyph is, the more often the glyph will cycle like this.
Buyers will bring some stability to glyphs and slow the cycle. Also someone liquidating leveling glyphs (they are not using QA anyway) will hold those down longer. But there are several lukewarm glyphs from research and books that will do this.
I do not expect every server to see this. For the longest time, I was the main cause of this effect on my server. Some things like this I don't do as aggressively anymore. But it was a nice way to passively reset the glyph market when I did do that. All it takes is one person to post by hand or use some other logic to post glyphs with and that cycle can be interrupted.