More than likely- grandad did it. 
naw- I bet she had some fear about this happening. Somewhere along the line, whether somebody said something, or she saw something on the internet about coffins being a hot theft item, perhaps even a tv show plot, or something, planted a seed of concern in her brain.
She had a fear. Someone could have stolen her kid's coffin, leaving her kid wet and cold beneath the earth. Never mind the coffin wasn't heated, so the complaint should have been simply "wet", or, "damn, it used to be just cold in here, but now I'm cold AND wet...and dirty..." , the fear was there, and wet and cold would have been the complaints.
Good thing I'm not buried, because I'd complain about everything. "Damn maggots eating my brain... Can't scratch my back... I'm hungry... Dark in here..." I'd be someone else's worse nightmare. They'd dig me up and kill me again to shut me up. But enough about me.
She worried. Over time, it festered and grew, reflected in the ever worsening spiral nightmares, until action was required to fix it, and it confirmed her worst fear.