An auction starts when there are multiple backorders on a domain.
This status of 0 bids shows that there have been no bids on the domain. Even though there are multiple backorders on the domain, not everyone with the backorder will choose to bid. You may have been the person that placed the first backorder and that is why you were notified of the auction. You will see many domains with 0 or just one bid.
For example scenario: You were first to backorder a domain, there were later three other backorders, and the domain then went to auction when it was caught by Sav. If none of the other three individuals places a bid at a higher price than the domain registration cost (plus any transfer or renewal fee), then you would simply be awarded the domain at the original backorder price. By the end of that auction, there have been 0 bids. Or in other words, no additional bids.
The auction is there to fairly sort out demand for a domain, and allows individuals a "second chance" to get the domain if they really considered it valuable.