My bet is the naughty neighborhood kids are throwing them. Kids will throw anything without first considering the consequences.

I have a lot of broken bottle glass that came along with the county compost I picked up. There is all sorts of undesirable crap in that stuff. I have not gotten any since the first and last time.

I have the reason it shows up in my yard! When I was having the ponds dug we came upon a dump in my yard.People use to bury it all.I had to pay $1500 in dump fees to get rid of it.
Unfortunately pieces of glass are still under ground and just like rocks up north keep coming to the top.After a rain is when I notice it alot!below is some crap we unearthed
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You could actually HEAR glass crunching beneath your feet!

I did find older pepsi bottles but none of the cool sasparillo or Lydia Pinkham types.I love old junk too,but perfer it not broken.

If at one time your yard had a septic tank or if a second drainfield had to be dug, the rocks could be from where it was dug up. Our pups were determined to dig out every rock that had been buried after our second drainfield. I guess they could smell them. The glass could be there for the same reason. In your case it is slowly coming to the surface from erosion.

Country nest said "broken but whole" huh huh