So, after PT_ helped me with my life counter program, I transferred it to my 128 gigabyte flashdrive.
After that, I went to pull it out, and when I did, it almost burned me.


Does anyone know why?

NOTE: It has been plugged in the computer for a little while, about 1-2 hours.

FYI: This is the flashdrive I am talking about

<oversimplification alert>

Well, circuitry (specifically resistance) creates heat (it's how toasters work, the heating elements are basically resistors, and it's called the Joule effect, IIRC), and your flash drive (circuitry) has been plugged into your computer (lots of circuitry).

From what I understand about flash drives, they are rather inefficient and most of the electricity that goes in gets turned into waste heat, which doesn't help.

Basically: energy flows in, not all of it is used (but it has to go somewhere) and it turns into heat.

Does this happen with other flash drives in that USB port? Does it happen with another USB port on the same computer? Can you plug in to another computer and see if it get's hot again?

USB drives are incredibly lower power. It's possible a PIN is broken on either the flash drive or that computers USB port that is telling the computer that this device needs more power (i.e. to charge). Since the Flash drive doesn't need to be charged all that extra power turns into heat, as iPhoenix said. So doing some generic troubleshooting (via the questions above) will tell us - and you! - where the problem is.

Every flashdrive and dongle (non-cable USB device) I have ever used has heated up over time. I think the reason we notice it for flash drives is because with most USB cables, the excess heat can dissipate along the cable, unlike flashdrives where it has nowhere to go.

It would be interesting if USB ports could be connected to a heat sink the same way other components are, but the heat doesn't build up much with cables and flash drives don't seem to be heat-sensitive.

It's expected to get warm. As iPhoenix and commandblockguy note, there's not a lot of dissipation opportunities for the heat generated in a mostly plastic enclosure. A flash drive under load may consume around 1W of power*. I also suspect that heat from the computer itself, especially if it's a laptop, will transfer via the metal port and add additional heat to the drive.

* http://www.anandtech.com/show/10163/usb-flash-drives-power-consumption-measurement-using-plugables-usbctkey/3

It does not happen with other flash drives, and it occurs whether or not I use the same computer.
It also occurs with all of the usb ports on all of the computers, although the effect is less pronounced if the port is not a usb 2.0 or 3.0 port.