I’m no injury expert but I have run on and continue to run on a lot of snow and ice. I would bet it has to do with the amount of slippage that you get when you are toeing off of the ice. It has a tendency to stretch that groin out a little farther then when you don’t slip. And more times than not it’s a real quick slip and kind of a pull on the hamstring that isn’t too noticable. Over time (4.5 hrs) I bet all those little slips can add up and do some damage.
What I do when I come up on some ice is slow WAY down and land squarely on my whole foot and kind of push off that way too.
Does this help at all? Not sure if I’m being clear or not.