I tacked onto starboard right in front of him — why am I in the wrong?

Under Rule 15, when you acquire right of way (by tacking, gybing, or because the other boat changes course) you must initially give the other boat room to keep clear. You cannot tack into a position where the other boat has no chance to respond.

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<p><a href="https://sailing.kiwi.nz/upwind/rule-15-acquiring-right-of-way/">Source: Sailing Kiwi</a></p>

Next on this shelf — The Upwind Beat