This is a known bug that has been very stubborn. I'm unsure of the exact circumstances that cause this, but I believe it's at least somewhat related to latency. It can occur on multiple powers, but Sniper Shot is by far the most common. Unfortunately the only workaround is to do respawn or transfer maps.
This same circumstance happens every two or three missions for my son in PVP.
Originally Posted by Vandrel-Blitz
This happens to me ALL THE TIME in PvP, very annoying!
Nearly every other PvP map I play on I end up with the sniper shot bug that roots my character in place until I either leave the mission/map or the map/mission ends and I leave. /stuck does not fix this at all and you will be force to sit there and watch your character stand still and have a lazer shooting off into no where.
This happens even after you die
While rooted you can't turn, can't move, nothing movement related but you can still attack, target and drop turrets, etc. but you won't see any animations.