It's probably due to the latency of when you get hit. On your screen it appears you're still moving forward (for like 1/10th of a second or more depending on your ping) yet server-side you've been hit. That delay resets your position when it reaches your computer, which makes you appear to be moving "one step back" .

Honestly, just don't cross traps with your shield. Most of the time it'll break, resulting in you taking damage anyways, or leave you with a near-broken shield which is useless if some monster actually hits you. I never cross traps because I've learned it's utterly pointless. Have some patience and cross when it's safe.