this seems to happen to me alot too, whenever im weed whacking in a high grasy i step around in it before i weed whack but still some dont leave until it's too late
I've had a very good year mowing my yard. No causalities. But then the mowing schedule is posted. My wild radixes usually go visit friends, take in a movie or just go shopping on mowing days.
You guys must have lots of garters if you have to watch for them when you mow. Good problem to have, i guess, if you like garters. I'm lucky if I see one or two garters a year. Never mowed a garter but I do watch out for the copperheads and cottonmouths down here. Ran over one's tail a while back and it started chasing me, thankfully my underwear was the only casualty that time. So a weed whacking accident would also be a bad thing, except I would be doing the dying instead of the snake.
I live in a highly urbanized area. No snakes in my yard today. I used to see them (very small T. ordinoides) occasionally back when I first moved in in 1992 but there was a field behind my house back then. Now it's a dental lab.