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.