Non-ECN Brokers must trade, one way or the other. They are always on the other side of every position made. That's automatically a trade. Now since there are a lot of such trades, it's all being aggregated. If there's an imbalance then the broker has a positional exposure. They can leave it or they can offset it in the market. If they do the latter, they are trading again.
You are, of course, concerned with them having that directional exposure. If that bothers you, trade through an ECN broker and pay the commission.
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