It has to do with lag. You only thought you got past them - but in fact they caught you several seconds earlier and it just took your computer that long to recognize you were snared. That's why you seem to 'fly' across the room all of a sudden.
Turning your computer's power settings to optimal can help, and also closing any unneeded apps or programs while you're playing Wizard101. Staying out of crowded realms is another option. And finally, lag definitely gets worse during busy times (weekends, afternoons, etc), and when Internet traffic in general is very heavy. So, playing during off hours can help free up bandwidth enough to let you sneak past those street mobs more easily.