I got a WL offer that was cancelled by the sender, sending me back to the waiting list. Now I'm #2, not #1. Any ideas why that might be? It's not too upsetting, really, but I'm wondering how it happened.

Someone could have come off vacation hold.

From the Help Docs:

Why did I move *backward* on my Wish List?

This happens when another member who was "ahead of you" in line previously  and was taken out of line gets back into line. 

This can happen if a member (or members) ahead of you in the Wish List line:

• Put her account on Vacation Hold (and this book was not set to auto-request).  This will temporarily move you up in line for the book, since Vacation wishes not on auto-request are hidden by the Vacation Hold.  When the Vacation Hold ends, the member goes back in line ahead of you.  This is a very common scenario. 
• Got offered the book but used the Not Now button - this puts the book back on the Wish List in its previous position, on hold.  If (a variable time later) the member removes the Hold from the Wish, that member will be ahead of you in line.
• Put this Wish on individual Hold on the Wish List, and then later unHeld it.
• Got an offer of the book, but the sender canceled it (did not mail it).
• Got an offer of the book, and the sender mailed it, but the book never arrived and it was declared lost in the Mail
• Received a "problem" copy of the book - damaged or wrong, etc.  This will put the member back on the Wish List in his or her previous position.

Please be assured that we would NEVER put another member ahead on the Wish List without good cause--in order to go ahead of others on the Wish List, a member MUST have Wish Listed that item earlier (on an earlier date) than they did.

In addition to the vacation hold that Margaret suggests, members have the option of putting WL books on individual hold.  I do this for a lot of my WL books that I'm not ready to receive yet.  I still move up in the WL line, but if the book is on individual hold, it's not counted in the WL line.  If I decide I'm ready for a book I have on individual hold, everyone behind me in line will be bumped back a notch when I take the book off hold.

Also, it's possible that something similar could have happened to the person just before you in the WL line, where their order was cancelled or the book was lost in the mail and they were put back on the WL just in front of you.

ETA--DG beat me to the post with a much better/more detailed answer

Another possible explanation is that the #1 person has a Requestor Condition that was refused by your Sender and the book was then offered to you.  RC's can sometimes cluster around the top of the WL. With the cancellation by your Sender, you & the #1 are back where you were to start with. 

Ah, I knew there was some way that made good sense. Should have checked the help docs... I forget they're there, from time to time! 

:)

Thanks!

With RCs, the Not Now button and the Wish List Hold, its is actually not too uncommon to be offered a book when you are not in #1.