I had accepted a WL offer and the sender had already printed the label with a mail by date of 30 AUG. Today I suddenly see the transaction has been cancelled and my credit returned to me. 

My only guess is that they "accepted" the transaction without reading my RC and before mailing realized it didn't meet my conditions.

I don't have a crystal ball, but they couldn't have picked a mailing date without first agreeing to your RCs.  It may be that they couldn't find the book (which always amazes me -- why accept a request without the book in hand?), or it fell in a puddle or otherwise became unpostable during the mailing process, or they've had some sort of family emergency come up ... lots of potential reasons, all of which will remain speculation.

I hope you get a new offer soon.

There could be any number of reasons. They were wrapping the book and spilled a glass of water on it, their daughter came up and said they wanted to read it, the cat or dog took a bite out of the book. They could have accepted before finding the book and realized they didn't have the book or that it wasn't postable. They could have suddenly had a family/work/life emergency and won't be able to send it. They ran out of the money for the month. They could just be a meanie and will randomly post and cancel books until the site catches them to boot them off.

MANY reasons why transactions are cancelled. Hopefully it gets posted again soon for you.

A number of things could have happened.

I once had already accepted a request & printed out postage but the night before I got the book packaged, I accidentally spilled water on it which crinkled the pages.  Originally it had looked like a new book but it didn't anymore.  I had to cancel the request, but since I'd already paid postage, I was out that money so I'm a bit more careful nowadays.

There could have been a number of things that happened though.

Things do happen.  Once I accepted a request for an old hardback book.  As I was inspecting the book before wrapping, the book separated from the cover.  I had to cancel.

That happened to me too, and also I had the second book in the same series listed, and I am darned if I could find it.  had to cancel 2 in a row, hideous.

I've accepted RCs before, then when I re-look at the book before packing it up I sometimes realize it may not meet the RC after all.

"They could just be a meanie and will randomly post and cancel books until the site catches them to boot them off"

LOL

Recently, while I was clicking to accept a request, my cat, Raven, was busy spilling Diet Coke on the book.  I clicked before I could react to her and had to cancel it about 2 minutes later, after cleaning up the pop.

Oops.  Missed the mail by date in the OP so that cancels my theory.

I've had to cancel books that I had accepted already.  One time I set a WL book on the kitchen table to remind me to mail it the next day.  At some point a box of babywipes got put next to the book.  Under the plastic lid thingy, one wipe was caught.  The wipe touched the book and the book sucked the liquid up like a sponge.

Another time I cancelled pending books to mail because my step-mother passed away and I just didn't want to deal with mailing books.

Got an email last night saying book was cancelled due to not meeting my RC, the book is smelly........guess I was right and they didn't read that before clicking.