I've recently found out the problem and solution. The problem is that you tapped the touchpad twice real fast. For example, you tapped the track pad with your index finger and then u tap it again wit your middle finger. This is like a glitch, i don't know why apple would make this so it has to be a glitch, and it drags everything.
The easiest solution is to do the exact same thing. Tap the touch pad with your index finger and release, and quickly touch the pad with your middle finger and release. Once your done, a small window will pop up that is the same thing as the right click window. After that the dragging should go away.
Another solution is to restart your computer. This always works, but its a drag.....
Good Luck!