Looks like it might be a ritual, in just a recent instance they told people in northern Gaza to go to the south but once they moved they fired at them: https://www.pbs.org/newshour/world/70-people-mostly-women-and-children-killed-in-israeli-strikes-on-gazas-fleeing-convoys-hamas-says

Hamas was created by Israelis and is infiltrated at ALL levels. Making that "surprise" attack is impossible, the border is really small and quite easily controlled by relatively few operatives. It doesn't take much to realize there is something else going on.