C'est ainsi que, le troisième jour, je connus le drame des baobabs. Cette fois-ci encore ce fut grâce au mouton, car brusquement le petit prince m'interrogea, comme pris d'un doute grave: ...
There are three verbs in the passé simple here: connus, fut, interrogea.
There are basically two factors here that explain their usage:
The difference between the passé composé and the imparfait.
The fact that in literary usage, at least 70 years ago when this was written, the passé simple is used more or less exactly where the the passé composé is used in speech.
The reason these aren't in the imparfait in this case is because they talk about discrete, finished actions.
connus refers to a sense of connaître meaning "learned, became acquainted with" rather than the more general "knew about".
fut is the hardest of the three, because "was" can be either a stative, ongoing verb or a one-time action depending on the sentence. In this case, "it was thanks to the sheep" in practice means "the sheep caused me to learn about the drama", which is a one-time action.
interrogea is because the Little Prince asked a definite question, then was silent for a minute. (He then asked another question. For this reason, you might be tempted to call it an ongoing conversation and use interrogeait. But in a narrative context, the use of the imparfait there would imply that while they were in the middle of questioning, something else happened — that's also addressed in the answer linked above.)
Once we establish that these aren't imparfait, it's the literary context that dictates passé simple instead of passé composé.
When you quote courus, I assume it's from this sentence later on:
Mais le danger des baobabs est si peu connu, et les risques courus par celui qui s'égarerait dans un astéroïde sont si considérables, que, pour une fois, je fais exception à ma réserve.
This one isn't a passé simple, but the participe passé used as the voix passive (the risks run by those who...).