Why Can Eating Ice Cream Too Quickly Cause a Sudden Headache?

This is not the brain literally ‘freezing’. It is a cold-stimulus headache. The International Classification of Headache Disorders describes brief frontal or temporal pain that begins immediately after cold food or drink passes over the palate or back of the throat, and usually resolves within ten minutes after the stimulus is removed.

The most cautious explanation is that rapid cooling in the mouth stimulates pathways of the trigeminal nerve, which carries facial sensation. Pain signals from the palate enter a system shared with sensations from the head and face, so the brain locates the pain in the forehead. This is referred pain. There is still no single settled account of why some people are more susceptible or exactly how much vascular change contributes. Small blood-flow studies have found changes but not wholly consistent results, so the familiar story of vessels constricting and then dilating remains a hypothesis rather than a complete proven mechanism.

Eating quickly cools a larger area over a shorter time and is therefore more likely to trigger the response. Slowing down and letting very cold food warm slightly in the mouth reduces the sudden change. The headache is generally brief and self-resolving; pain that persists, recurs unusually or occurs without a cold stimulus should not simply be labelled ‘brain freeze’.

https://ichd-3.org/other-primary-headache-disorders/4-5-cold-stimulus-headache/4-5-2-headache-attributed-to-ingestion-or-inhalation-of-a-cold-stimulus/
https://pmc.ncbi.nlm.nih.gov/articles/PMC10144624/