It's a bug from the Vehicle Cam mod.
"Known bugs
# Look behind in car uses on foot control.
# Drive-by shooting cannot be performed unless you restore default vehicle camera (Press Change camera button).
# Weapon crossair appears on screen when you change radio stations.
# Camera might freeze after some mission cut-scenes.
# Exit your vehicle first if you want to restart or load other savegame, because on foot camera will be adjusted."