Telogen effluvium is the medical name for a burst of hair shedding that happens when a stress on the body — illness, surgery, childbirth, a crash in calorie intake, or rapid weight loss — pushes an unusually large share of hair follicles from their growing phase into their resting ("telogen") phase at the same time. Those hairs then fall out together roughly two to three months later, which is why the shedding seems to come out of nowhere.
This is the mechanism behind most hair loss on a GLP-1: the medication itself is not thought to attack hair, but the rapid weight loss and reduced protein and micronutrient intake it can cause are classic triggers. Telogen effluvium is diffuse (thinning all over rather than patches or a receding line) and usually self-limiting. Shedding typically slows within about six months once weight stabilizes and nutrition is adequate. See hair loss on a GLP-1: what the evidence actually shows. The American Academy of Dermatology lists losing 20 pounds or more among common triggers and says hair tends to regain its normal fullness within six to nine months (AAD (external link)).