I reached the layline first and tacked onto starboard — so why did getting hit disqualify me?

Tacking takes you through Rule 13 and then hands you right of way — and Rule 15 says you must give the other boat room to keep clear. Tack too close to a boat fetching the mark and the collision is on you, even though she hit you: you broke Rules 13 and 15. She is exonerated under Rule 14 because no boat has to anticipate that another will break a rule. (World Sailing Case 27.)

<iframe width="560" height="315" src="https://www.youtube.com/embed/RiFRbTJcpk8" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/windward-mark/case-27-tacking-too-close-at-the-mark/">Source: Sailing Kiwi</a></p>