I know that Go Daddy, NameJet, and Snapnames are well aware of domain investors who reach out to the owners of expired domain names in an effort to purchase them before they go to auction. This probably annoys the auction houses and it has spurred a debate about this practice on forums and in private discussions.
The rationale for domain investors doing this is simple. The domain owner may not have realized the domain name was worth anything, so a lowish offer could get the owner to renew the domain name and then re-sell it to the person who inquired about it. The buyer would then get to purchase a domain name in private before others had the opportunity to bid on it. If others pre-bid on the domain name, the buyer would know that there is interest from other parties.
There are a few downsides to this practice. For instance,