To be certain means to be sure of something. For anything to be of a particular reason means that the reason is more distinguished exceptional

I think your question might get clearer when it is talking about the difference between "certain circumstances" and "particular circumstances" I'm not a native speaker, so please just take this as my opinion. For example, "XXX occurs under certain circumstances" means XXX happens under the kind of circumstances in which all the requirements are met. Think about how a volcano erupts. I'm not a geographer, but let's say it happens when the temperature of the magma in the volcanic chamber goes up, the magma pushes the top of the volcano with strong pressure, and it breaks the "lid" of the mountain and spews a lot of lava. So, you can say "when all these occurrences (or in the above example sentence, "requirements") are met, it erupts," or, it erupts under "curtain circumstances" (circumstances that are dependent on some requirements). "Particular," on the other hand, is more specific and stresses like "THIS IS IT" kind of sense. Like, if my girlfriend said "I want a particular ring" that means she knows exactly what type of ring she wants. This sentence makes sense because the imagery of the word "particular" is like you are pointing at something with your finger, saying "THIS IS IT!!!". So, you know what it is. But the word "curtain" doesn't have this sense. It doesn't stress the "THIS IS IT" sense. You are not pointing at one thing, which makes the sentence "I want a curtain ring" sound unnatural because it is like you are not specifying the ring you want. It's not in your mind, nor is it in anybody's mind. It's unclear. Does that make sense?