Leaving playing cards on the table overnight

Hi people - new guy in town. Stumbled on this site with a question I've had for as long as I can remember,

My grandmother always told us to never leave playing cards on the table overnight. Whenever we asked why, she would grab us by the shoulders or by the head and snap "JUST DON'T!" or "Because I said so". With an answer like that, needless to say we didn't ask often. She never gave us a reason why, and nobody else in the family knows why she was so intense about this one thing. Especially since she was not a superstitious person, or religious for that matter.

Her emotion while telling us this almost seemed... fear. I was wondering if anybody has heard of this, if this is a superstition or just a crazy granny's silly rule.