Learn Russian: Это случается редко.

The -ся ending makes this a reflexive verb. In Russian, actions that just "happen" on their own, without a person doing them to an outside object, often take this reflexive ending.

Why do we use Это here instead of Этот or Эта?

We use the neuter form это when "this" acts as a standalone subject referring to an abstract idea, a situation, or a general fact. You only use этот or эта if you are pointing to a specific masculine or feminine noun, like "this table" or "this book".

Why use the imperfective verb случается instead of its perfective pair случиться?

The adverb редко (rarely) tells us this is a recurring or habitual situation. In Russian, you must use imperfective verbs for actions that repeat over time, even if they repeat rarely! The perfective form would be used for a single, completed event.