Maybe this has something to do with the cost of gas dropping? One used motorcycle dealer I talked to a couple of months ago said that when gas prices hit $3 everyone was looking for cheap commuter bikes (which he used to justify asking $3400 for an average FZR600).
I've noticed the price drop around here too. The problem is I need to sell my bike to take advantage. The best bet is to sell mine during the riding season to get the best price. buy right before christmas. But that leaves me sharing my wife's bike during the best part of the season. Dang.
Probably no one thing causes this if it's really an overall trend. But the greater the sales of new bikes, the greater the supply of used. My guess is that new sales are the biggest factor, though the other suggestions sound good as contributing factors. You know, much the same thing has happened in the used car market.
I expect BMW & HD bikes to do well at retaining value as usual. Ditto some of the more desirable models.
Back if Feb 05, I almost pulled the trigger on selling The Fat Lady, my Honda ST11. '98 with all maint records, fresh Metz rubber...blah, blah. I was offered $5000 on trade and advised to ask about $6000 - $6200 if I sold myself. Looked at the whole situation again last month (I can't get that gosh darn XB12X out of my head). Down to $4000 on trade. Hmmmmm. And I see what appear to be clean ST's now selling for low 5's.
Newer SVs sometimes have trans problems downshifting 2nd-to-1st. Get up to 3rd or 4th, downshift all the way to 1st like you're stopping at a light, and listen for any screwdriver-in-a-fanblade type noises.