How is Broker Protection Clause, Invalidated?

[Jim Witten]

I thought previous listing brokers are protected regardless. That's the whole point to prevent a seller from just swapping listing agents or firing listing agents.

 When an agent shows "SARAH" the property, "Sarah" is now part of that agents list of names. So if "Sarah" later buys the house down the road, the original broker still gets a part of the commission. Because he was the procuring cause right.

Why would seller hiring a different broker invalidate any of that protection to the original broker?