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.

If the break is before the meter, it wouldn't be on the bill. Nevermind the bill, in a Long Island home with a metered leak, you'll actually hear the meter buzzing away in the basement.

Weather is pretty unlikely, Long Island generally is not prone to sink holes as it lacks the "soft bedrock" geology that leads to them. Long Island is basically a long, extended sandbar that was pushed up by a glacier, sand and tumbled rocks, there just isn't any kind of limestone or carbonate bedrock to cause sinkholes

Long Island homes (with city water) usually have the water meter in the basement right next to their shutoff.

As far as Seattle... well, I don't know a damn thing about Seattle, or their water hookups.

The real question is: where is the leak, and what is responsible? Was it the developer? Was it the water district? Was it knicked by a landscaper or utility worker trench and reburied?

I'm sure we will find out in due time