Asides from the FA having a mood on, the only other reason I can think of, especially if nothing was said to the other at-the-gate recliners on your flight, is that the person(s) behind you, while you were sleeping, said to or signaled the FA that you were encroaching in their space too much too early. They didn’t want to confront you directly, so they asked the FA if he would. Much like I don’t want to contact the people in the room next door to me in a hotel myself if they’re too loud late at night, but will phone the front desk and ask them to deal with it. If you were sleeping, you wouldn’t’ve known. Rather than give the people behind you up on their request, the FA did the “it’s regulation” thing to keep them out of it.

Pure conjecture on my part, FWIW, and doesn’t make it right. He could’ve simply asked you to move your seatback up a bit.