Well, the most are clearly from your last reason. But the other reasons are what open up those spots. For example, one kid at Harvard gets cold feet & backs out. So, they accept someone from the waitlist who was originally going to Chicago. Now, Chicago has an open spot & takes someone from their waitlist who was going to Northwestern. Now Northwestern takes the guy who was going to BU.... & it trickles on down the line. So, one person's cold feet can open up waitlist spots for a dozen other people.
Also, to your list including cold feet & laziness I would add 1. people who get job offers late in the game & decide to accept jobs rather than go to law school. 2. People who realize that their previous financial plans aren't going to work & defer for a year while they straighten out their monetary situations 3. People who experience a trajedy (like death in the family) & decide that they can't handle school right now... & I'm sure there are a bunch of others. The point is, not all people who back out at the last minute do it because they're nervous about the workload or lazy.