rear Shimano WH-RS100 drive side spoke broke, causes?

[Ajax Bay]

A spoke broke on my Shimano WH-RS100 rear wheel, drive side, about two inches from hub. , , , am just concerned there's something more fundamentally wrong. Or is it just something that happens after a while?

To help with Q1
Spoke failure is not infectious between wheels! The other one you've fitted will hang on its own spokes.
https://bike.shimano.com/products/components/pdp.P-WH-RS100-CL-R.html
This wheel has 24 plain gauge spokes.
You describe the spoke as having failed 2" from the hub. This is unusual: these spokes normally fail through fatigue after millions of revolutions, if ever - at the (work hardened) bend by the hub flange.
I recommend you remove the cassette and carefully inspect (feel) the remaining right hand side spokes. I suspect your spoke parted where it had been damaged by a chain overshifting inside the large sprocket. I cannot think of another reason for a spoke parting there (part way up its length). Report back.
Anecdotally, I recently went to my LBS to retrue a wheel on their jig and during that a spoke parted on a tensioning twist of my spoke key. I had damaged half the spokes (either the trailing or the leading 8) in the manner described above (but do not remember ever doing so). I bought 8 spokes and replaced/relaced the lot.

 

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You describe the spoke as having failed 2" from the hub. This is unusual: these spokes normally fail through fatigue after millions of revolutions, if ever - at the (work hardened) bend by the hub flange.
I recommend you remove the cassette and carefully inspect (feel) the remaining right hand side spokes. I suspect your spoke parted where it had been damaged by a chain overshifting inside the large sprocket. I cannot think of another reason for a spoke parting there (part way up its length). Report back.

There is some damage on a number of the spokes from the chain coming off of the cassette and going in-between the cassette and the spokes. Some grazing as it were. It's not too significant. And that grazing is only about 1-2cm from the spoke ends. There's no other apparent chain damage anywhere else on the spokes, none around the break. The point where the spoke is broken is definitely quite a bit further out than the largest cassette cog. I've just measured the broken spoke, the break is actually 3 inches from where the spoke attaches to the hub. And the spoke which has broken has no damage on it from the chain coming off the cassette as it is one of the spokes coming from the other side of the hub rim so is protected from the chain by the other spoke just near it. The rest of the spokes' tightness feel basically fine, nothing clearly wrong. Thanks.