It is really just speculation on our part; rarely do applicants tell us the reason for requesting another date. What really burns is when they cancel the January interview a few days in advance and we cannot schedule another applicant to take the spot. Not only do they cause us to have an unfilled spot, but they deny a fellow applicant the opportunity to interview at a program that might have been their top choice.
I will issue the first plea of this cycle: when you know that you are no longer interested in a program, withdraw immediately. Do not wait to get an invitation to interview, do not wait to get an acceptance, do not wait for the second look weekend, do not wait until May 1st. While it may be gratifying to rack up a lot of acceptances and have programs pay you a lot of attention, you jamming up the works and are screwing your fellow applicants. This is not directed at you in particular, Elephant, but to the individuals out there who will have multiple acceptances and will hold most of them to the end of the cycle. Back when the AAMC allowed us to see an accepted applicant's other offers, I would know who these people were and I would suggest that they decline our offer sooner rather than later. Sometimes they did, but more often they would hold out until the deadline and then send a generic email telling my how difficult the decision had been (though they did not attend the second look weekend or initiate any contact with us), and that they hoped we could offer the position to someone who would find our program a better fit (I would have been able to offer the position to that poor sod back in February if you had declined once you got into your dream school).
Sorry for the rant; I'm off my meds today.