OS X V10.6 Snow Leopard :: 2nd Monitor - Icons Move To Edge (sometimes)?

I have a 2nd monitor attached to my MBAir.  I have icons (folders, a few files, and some aliases - not a lot!!) positioned on both monitors.  I can work this way just fine for days.  (Yes, I do restart nightly and, yes, after a basic restart all the icons stay put.) But sometimes, out of the blue, something happens that causes all the icons on the 2nd monitor to line up on the border between the two screen displays (i.e., almost off the 2nd monitor but not quite completely onto the MBAir's display).   

This seems to be related to saving a file onto the desktop -- say, when I'm using Safari on the 2nd monitor and I drag the address bar address to save a webloc file.  However, I can do this more often than not with no odd effects.  (However, lately I've started dragging into a FOLDER on the desktop to avoid this.)  Now, I just watched it happen when all I did was save a tiny .TXT file to the desktop. Of course I can fix things by dragging everything back to where it was (plus, I typically do a Restart before that since I figure something has gone amiss with the system so I'd like to clean things out).  But what I'd really like is to figure out is why this is happening in the first place so that I can prevent it ... or, at least, so that I can stop doing whatever special behavior is causing it. 

Info:
Mini + Air11, Mac OS X (10.6.8), iPhones, iPods, iPad, MacBook Pro (10.6), PB G4 (10.4), ATV