squash borers--the limited googling/u-tubing I did suggests they are over-wintering in your soil, and recommendations included: watch for the moths in early July (depending on location), monitor plants daily for eggs during this timeframe, use cardboard tubes stuck in the soil for the seedlings to grow through, and possibly wrapping the first few inches of stem with foil.
Once infestation is suspected, try to get the vine to root further along its length, and use a needle to poke along the suspected borer site.
I managed to find 5, not all my plants were infected as first thought, but I have all the remaining ones rooting along the stem just in case.
I saw a list suggesting some squash is more susceptible than others--will post if I find it again.
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