alt Hacker NewsGoogle almost certainly uses stuff in your gmail to recommend ads to you. I had a very, very specific thing that was ongoing for a year or so and as soon as I sent someone an email about it last week, a lot of ads starting showing up for that very specific, not common situation on YouTube.
Maybe the person you sent it to Googled it almost immediately. Then the gmail meta data linked you to the specific thing and that association followed you to YouTube.