When we (users with enough reputation) protect questions, the standard text talks about "low quality answers", and it's a frequent thing that is done with very old questions which are resurrected to the front page either due to a new answer (like yours in this case) or because the question was linked offsite.
These scenarios tend to lead to many low-rep ...
Your own list illustrates how pointless this sort of proactive activity is: in most cases, few or no new answers were posted regardless of whether the questions were protected. Indeed, the most-answered question of the lot was immediately protected.
It's not the raw traffic that creates the problem. It's traffic coupled with a question ...
This is by design:
Users with 10 or more reputation can answer a protected question. However, the +100 account association bonus is ignored for this check, so you must have earned 10 or more reputation on that specific site to answer a protected question
Auto-protect questions that get more than N answers from new users in a 24-hour period
What happened is that two of the people answering got fairly quick upvotes on their answers. Enough to go over the 10 rep requirements.
At that point, when a new answer comes in, those are excluded.
This is working as designed - may need tweaking though.
There are two deleted answers on the question. The later one is complete nonsense, and I'm guessing is what triggered the protection.
That said, I do agree with you the protection here was a bit premature. Protecting a question goes against our promise that asking and answering is completely free and requires no rep. It's a last resort tool for questions ...