Like Pockemonster said, you're striking the strings more than plucking them.. sort of using the your fingers like hammers.
However, it seemed to be only when you were playing faster.
When you played slow, it was more relaxed, and you were pulling, or "plucking" the strings in a more controlled way.
Just slow it down, and build up the technique using the same approach as you did when you were playing slow.
If you make contact before you start the movement to play the note, then its almost impossible for the strings to bounce on the frets like that, because the string begins its motion moving -away- from the frets. Even if its touching at the start of the note, it can't rebound far enough to hit them. If you hammer into it like that, then you're slamming it toward the frets, and that sound is what you get
