Thanks to the stealth and switching directions, willbender had to use his mobility skills to catch up. At this point, he should have given up, but he didn´t.
Edit: BTW scrapper is no the only class that can stealth and kite away. There are classes and specs that can do that far better. Shortbow thief, sb/druid smokecalse/greatsword combo, mesmer stealth and blink, mechanist elixier S, rocket jump and teleport.