It is very common both on snapnames and namejet that pre-release domains suddenly disappear.
I think sometimes the smaller registrars send their domains to auction at snapnames and namejet and when they see that the domain gets many bids they pull off the domain from auction and keep it for themselves. So it is not always the previous owner who renews but sometimes it is the registrars.
You might say, then why do the registrars send those domains to auction in first place? Why not keep it to themselves from the beginning on?
The answer is easy. They don't know what domains to keep. They use the domainers to sort out the quality domains. There are thousands of domains. Just look at the numbers on this page: namecatch.com/user/stats
There are over one million domains on pre-release auction right now.
I don't know what percentage snapnames gives the registrars but if the registrar keeps the domain to themselves then they get to keep 100% of the value instead sharing it with snapnames.