Some places that are not ranked as well tend to send acceptances earlier than some of the better ranked places in an attempt to lure you on early. So, I would expect this type of thing to be common.
This exact thing happened to me. I had already booked tickets for some schools that rank lower down on my list, and am just now hearing back from some of my top choices. It's tough to wait it out when schools are asking you to choose a visit date and book a ticket as soon as possible. I still think it's valuable to visit the schools, knowing that their rank in your list may very well change -- like TakeruK mentioned -- but it's definitely difficult to coordinate the way offers are so spread out.
I'm glad I'm not the only one in this situation! I wanted to wait to buy plane tickets for the visit, but my parents pressured me into it, saying they would be too expensive if I waited.
I should really stop being a child and trust my own instincts.
Oh well. I guess at this point my option is to just be lame-o and plan a visit to Cornell on an alternate weekend (so sad to miss the real visiting weekend though...) and hope no other schools I get into have conflicting dates! Best of luck to you blackmass!