Simple.
Higher speed = more vibration and more air moving inside the HDD.
What does this mean?
Sectors are written in tracks, the density is consistant from the outer edge to the inner most tracks, but the amount of sectors per track will shrink as you move inward more (because there is less circumference as you move inward)
This is also why the outer most tracks are the fastest, thus your OS and games written on the out tracks =)
(you can check this by running hdd tune, watch you MB/s slowly drop as it moves through the LBAs, this is because the higher the LBA, the further in the heads are moving)
Ok so, I mentioned density. To make hard drives larger, they are cramming more sectors on a platter, to do this they are making the magnetic domains "sectors". smaller then ever. this also means that the tracks have more sectors per track, and are thinner then before.
The heads are far from perfect, inside when the platters are spinning there is significant air hitting them, this is actually how they manage to not touch the platters, on the heads there is a component called the slider (little black thing) looking at a high megapixel pic, or a microscope you will see aerodynamic ridges in it, used to catch the air and float above the platter surface at ~4nm)
Ok so the wind is knock the heads around, making it hard for them to stay on track. Now throw in some vibrations, even subtle to our touch, but very large in scope of a hard drive (sectors are very small. you would need a unbelievably powerful microscope to see a sector)
Hard drives implement a "Closed loop system" to help with this. It's called Servo. Servo is written at the factory after the hard drive is fully assembled. it is written on the platters and can never be changed. it is written by a servo writer, it enters the hard drive through the side, notice the silver rectangular sticker on the side of the drive covering a mysterious opening, now you know why its there.
The servo utilizes "gray code" - you can google that.
This gray code is a signal, it is written in between the tracks, and is used to tell the heads if they are staying on track. Without going in to too much detail, a very basic expanation would be to think of bumper bowling, with the bumpers to make sure the bowling ball stays in the lane, its kind of like that, only the bumpers are two different signals, if the heads sense too much of a certain signal then it adjusts itself to be in the middle again.
It really is much more complicated then this, but its too much to type in a single post, maybe I will write a book someday.
I hope this sort of clears things up for you, but it probably just makes everything more confusing. I was really trying to keep it simple =)
Enjoy,