You tried to turn and accelerate at the same time beyond the limit of rear tire traction. In other words, you broke the rear tires loose by accelerating hard in a corner.
Then, once the car started to rotate beyond the turn, you got behind in your steering and did not react fast enough to "catch it". Net result, you spun it around.
Nothing that a few track days can't teach.
And the 911 isn't worse, it is just "different". The Boxster rotates much easier and faster but is generally easy to catch using a majority of steering inputs. That's why people say that its "easier" to drive at the limit. But you need quick hands in a Boxster or you'll spin.
On the other hand, the 911 rotates more slowly but that rear end has a lot of momentum so it takes a lot more effort/skill to halt the rotation and reverse it and usually requires both steering and throttle inputs to do it right (which is technically more difficult than just steering alone).