My brother does this job and occassionally comes home plastered in frog or hedgehog 'smoothie'.
I know there is a particular graveyard he cuts where he has to go round on foot carefully first coz a deaf cat likes to sunbath there!
Generally those big mowers/strimmers make enuf noise, just sounds like a truly horrifc accident. Although I dont get how 2 healthy cats couldnt of gotten away beforehand unless they were under it when it started up?

Could the cats not have been playing in the field and ran out in front of the tractor at the wrong moment?

I doubt it would have been a dog attack as a dog probably wouldn't be able to catch both cats and cause those particular injuries.

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R.I.P Buster - Causing mischief over the bridge, sleep tight soldier