ZombiHendrix
There's a history behind it, but nowadays, it's just a scapegoat for our F-ups. Our pride won't let us admit the times when we fail, so we have to point the finger at Friday the 13th.
Yeah, that's a dumb reason for it. Also untrue. Ankh already pointed out at least one reason why folks these days consider Friday the 13th bad luck. The rest is just memetic mutation as the reasons behind it got lost and murky. And then there's all those movies and shows that have come up in the last century that portray it as a day when malevelent things come out and actively try to kill you.