You step out of theory into positions you have not understood, conceding the centre before move ten.
You left the opening worse off — behind in development, a damaged pawn structure, or a piece stranded on a bad square. The opening has exactly three jobs: get your pieces out quickly, fight for the centre, and tuck your king away by castling. Develop a new piece nearly every move, don’t move the same one twice without a reason, and don’t go pawn-grabbing while your pieces sleep at home.
The good news: it is a pattern, not bad luck — and patterns can be trained. Step one is measuring yours: how often it appears in your real games and how many rating points it drains. Then the daily repair: 5 minutes on your own positions, not invented puzzles.