could be the plan was to keep sky in yellow (they (sky) made it pretty clear early on they were quite happy to let the break have it) and make it a tough day and perhaps make up some time.
I doubt it. This whole “make them defend the jersey” thing always strikes me as received wisdom bullshit. As a GC contender you would always want to have as little deficit to the lead as possible to avoid too much negative impact from mishaps so why would you intentionally NOT be the leader? You can’t possibly know how the race will pan out with crashes, mechanicals, riders fading late race etc so it’s just rank idiocy or bravado to suggest you will take the jersey on a specific day.
My personal feeling was that BMC wanted to thin out the field and try to take yellow or at the very least take time from the other favourites but all they did was burn their team. In reality though, unless Porte couldn’t stick with the other leaders, there was little risk as today isn’t another mountain stage.