I've been watching a video on learning to sail, https://www.youtube.com/watch?v=k6DEToYI-7w&ab_channel=ClintonLakeSailingAssociation and much of it is clear, but it has left me with a couple of questions.
There are terms and techniques for sailing at various angles to the wind. Conceptually the simplest is a run straight downwind: you orient the sails sideways, and the wind gives you a straight push.
The instructor (around 20:00) recommends newbie sailors avoid doing this, going so far as to recommend that if the wind happens to be blowing in exactly the direction you need to go, you should zigzag somewhat to avoid having it directly behind you. The given reason is that a shift in the wind could send the boom swinging across the boat.
But it seems to me that for this to happen, the wind would have to shift fully ninety degrees, which seems unlikely; if we should be prepared for arbitrarily large shifts of wind direction, then by the same token the recommended broad reach is also not safe.
Is it the case that, counterintuitively, the wind is much more likely to shift 90 than 135 degrees? Or is there something else I am missing?