It appears to be based upon your timeliness replying to requests, reviews, sending photos.
It wouldn't surprise me if there is some subjectivity involved. For example, before I booked a stay I was #2 in the list, sometimes #1. I think when they review your profile (before it goes live) they apply a "potential" score and which gives you a boost.
From my perspective, I think photos carry more weight than how many requests you accept. I only take dogs from one household at a time. So, I archive a lot of requests as "not available." (Dog owners don't seem to consider, or trust the calendar's availability. I'm constantly getting requests when I'm already booked for one household's dogs.). So, in my case, I am consistently at the top of the list, yet I decline more requests than I accept.
But, I do a lot of photos. And, I reply 10-60 minutes. Even when my reply time dips to "within an hour" I'm at the top of the list.
So, either the photos are giving me rank. Or, there is some kind of subjective value they apply to a sitter's style, credibility, etc?