Every year we have the greenroom stories. It gets more attention when it happens to big names such as Warren Sapp or Aaron Rodgers (and more recently Geno Smith), but we see players stay on the board longer than expected literally every year and across many positions.
There are many reasons for a drop, but it mainly comes down to the fact that it's really common for there to be wide splits on how a player is rated among different teams. It's an old saying that the market on a player can be just one team willing to take him high. And if, say, just one or two teams have a player rated as one of the top players in the draft and neither selects him because someone they liked more was available -- a big drop can happen, and that's even before you factor in need. I know for a fact that there are some wide splits among teams regarding this year's QB class, and I'm ready for something that we'll later call a "drop" to happen.
I was asked to consider worst-case scenarios for some players on both sides of the ball, and the players were specifically picked because they generate a wide range of opinion.