Vasovagal syncope. Happens several times weekly in any large hospital's phlebotomy or vaccination clinics. It's a stress-induced response to seeing your own blood or having a needle enter your body and has nothing do with what was actually injected.
Some people even have a few convulsions after they pass out. Not pathologic at all, and they quickly resolve without any damage. Unfortunately, I can almost see the next click-bait, ill-informed headline write itself: "Patient receives COVID vaccine, immediately has seizure!".