I'm attempting to take the weekend off, as it's Labor Day here in the States and I have a bad habit of trying to work 7 days a week. So, this isn't "work".
At the basic level, the "why" isn't very important. It's only knowing that it "will" in the future. Each Key S&R develops for a fairly large set of reasons. When we're looking at the Spot FX chart, we're only seeing the current Price in the Spot market. Spot makes up somewhere under 50% of the volume in currencies. The massive part of the market is actually Bank to Client deliveries. That's why market moves are engineered, as the Banks actually take on the Risk from their Sales to Clients, and unwind the amount at a more profitable price.
That doesn't even get into Futures, Swaps, Options, Forwards and I feel like I'm forgetting a few other financial products. There is simply a lot more than Spot Orders stacked up there.
So a set of major actors were collecting Orders at that level previously. So after they ran that level (collecting Orders above it), they let the pair stay there so they could collect more Orders. Once they had their Orders, they moved it back down when the News gave them cover to get the pair moving again.
So, in this case, it's more that 1.6740 was their launching point, after they had collected enough orders before the BoE Governor speech. All of the big moves are engineered; we shouldn't care much exactly what's so important about 1.6740, just that it was. With the information that it is, we're watching what happens to it. In a Trending environment, the "Slam Through, Hover Back, Collect and Slam Again" method happened to be the choice for this one. It's normally going to be a sign of the value of that level.
It's also good to remember that there are massive funds that trade off Weekly & Monthly Charts. There are massive funds in certain Forex markets. Never forget that bit. But we don't really care why they're up to what they do. We just want to ride the Wave with them. That's how you make the huge gains.