Maybe granny was just a regular person before that.
I mean it kinda makes sense like, Slenderman drives granny insane, She kills slenderina,slenderman gets kicked out (or killed) And you wander into her house with maybe freinds or your just walking home and stumble inside and she either kills your freinds with the bear traps or she just kidnaps you.When i think about it this is kinda like the new resident evil's story.