It was probably the same as the fall detection algorithm. Maybe it detected a high drop that didn't move back up (if they were on the ground digging) .. although it should have canceled with movement? Depends how that software was designed. From what the comments say, it seems like the Apple watch only does fall detection (and only if you're over a certain age by default)

Can confirm, grandfather died from a heart attack when shoveling snow.

Mine slipped a disc while shovelling as well. The torso twisting motion is really bad if you have existing back problem.

Any time I see someone shovelling a sidewalk or driveway with something other than a sleigh shovel, I'm baffled at why they're not using a sleigh shovel.

So it watches your bloodpressure or something?

I figured it was the jerking-around. Maybe it looked to the watch like she had fallen down.

It watches the heartbeat, probably arrhythmias specifically. You can't monitor blood pressure with just a watch. You need a blood pressure cuff for that.

The Apple watch can look at blood oxygen levels too which might have been what triggered the alert.

Does it watch heartbeat via the gyroscope? If so then that's a darn sensitive gyroscope.

Via an optical sensor that can perform photoplethysmography.