It's basically to memorialize their Facebook ID in memory of those who died. But some people are abusing it for some technical bug of Facebook. Although not everyone has such an idea about this, for those who do resources with Facebook Technical Issue, it is an old trick to memorialize the ID of others.
But you have to keep your ID safe from this. For that, you need to change some security and privacy.
** Remember, your profile cannot be memorialized without someone from your friend list. So if it is Memorialized then someone on your friend list has done it **
Then let us know what to do
1. Turn off your timeline for now. Don't let friends post to your timeline.
2. You need to add Legacy Contact to your profile.
Go to settings> Personal Information> Manage Account> Add A Friend
Remember to add someone you trusted here. The only one who can memorize your profile when you die.
3. Turn off the public comment.
Correct these three things, for now, then you can protect the ID from this Harassment.
Your ID will be a problem because of you. People who do this kind of thing do it for fun, but it's not really fun.