Definitely this.
The brain doesn't consider our hair to be a priority, so when stress hits us (losing weight, surgery, stress, etc. . . ) hair is usually the first thing to go. Instead of directing nutrients to promote hair growth, it's now being redirected to another job in your body. Hair loss after a major weight loss event affects people in different ways. Some will lose a little, some will lose a lot (like I did when I lost 90 lbs), and some won't lose at all. It's a horrible genetic lottery.
It may sound a little dire, but the only thing to help you prior to this event is to be prepared for it. Make sure you're getting the protein amount recommended by the doctor, drink plenty of water, and get your vitamins in. If your hair is still falling out in clumps a year out - then there could be a nutritional aspect to your hair loss. Find a shampoo/conditioner set that builds volume and learn that you don't need to wash your hair every day. Manipulating your hair to make it seem fuller will be difficult, but the hair will grow back.
Me? I mean, I lost a ton of hair back when I lost 90lbs through the keto diet. I am not looking forward to losing it again, but I've talked with my husband and will likely shave it off + find a really nice quality wig. I've been wanting to shave my hair anyhow, so it may be the perfect opportunity.