A mod or 15k+ user manually selects a question for protection (this is an uncommon occurrence, and is not done lightly)
When a mod deletes or converts an answer (usually because they were intended as comments) by a <20 rep user, and there are two other such deleted answers on the question (for a total of 3), the question will automatically ...
Community protects questions that get three deleted posts from new users (9 or less rep). In this case, the question got three "me too!" posts. Two of these were deleted by moderators, the third (apparently) through the review queue.
After the recent changes to protection, a question can be protected automatically if it fulfills any of these criteria:
3 answers from new users deleted – this handles questions that tend to attract large amounts of spam over time.
5* answers from new users scoring <= 0 posted in the past 24 hours - this handles questions that are somewhat topical,...
"Highly active question" is a fairly new wording for protected questions. In general, questions are protected if they receive a high number of low quality answers.
In this particular case, the Pokemon Go question was automatically protected by Community after a third answer was deleted. The Cities Skylines question has no deleted answers.
As I don't haver 10k rep I can't see deleted posts I can't tell for certain, but probably because it got a couple spam/thank you posts.
If my memory server me right I've read somewhere that after three deleted posts, community will automatically protect the question.
15k rep users can also protect a question that gathers spam answers.