I sell Mann Co. Crates on the Steam Market, putting up 20-30 at once every now and then (I have probably 120+ on the go right now). Every now and then, Steam takes my oldest bunch of crates, removes them from the market, and re-drops them in my backpack. Why?
I've had this happen to me all the time when Steam's servers get overloaded during the holiday sales. I put up an item for sale, a few minutes later the listing is cancelled and the item is back in my in inventory. When it's really bad, I have had to do this over and over before it sells.
I suspect you're running in the same issue I did, and my guess the underlying problem is with database transactions timing out. In order to prevent market items from being duplicated or lost accidentally the transaction has to be done atomically and this makes it rather expensive database operation. Since crates seem to be a popular item they may be subject to high degree of database contention. If a transaction can't complete fast enough because of this then it will end up getting cancelled. Apparently Valve's developers think the best thing to do when this happens is to dump the item being traded back in the inventory it came from and not relist the orders.