Yeah, it's all conjecture as of now.

My uninformed but kinda smort guess is that it was a quick, massive failure before the meter, or possibly with irrigation that was on an off meter well (a common setup here). Long Island, being a sandbar, gives way to water pretty easily, so it would be quick work for a 1.5" high pressure main to excavate that cavity. I once saw very large main at a horse farm that left a crater big enough to hide a school bus in the ~30 seconds it took to shut down. If this was a normal residential main running all night, this size hole is totally possible. Because it's a grass yard, and a frozen one at that, the grass roots holds up the top 6" and viola, you have a hidden deathtrap.

If I'm right, her sand got pushed down into the water table, or gushed out all over her yard on the edges.