Learn Russian: Катастрофы случаются всё чаще. - Disasters are happening more and more often.

In Russian, to express a progressive increase ('more and more...'), you use the particle всё followed by a single comparative adjective or adverb. You never repeat the comparative word. So, всё чаще translates the concept of an ongoing increase in frequency without needing to say чаще и чаще.

Could I say всё более часто instead of всё чаще?

Technically yes, but всё чаще is much more natural and common. Russian strongly prefers the simple, one-word comparative form (like чаще) over the compound form (более часто) when a simple form exists.

Why does случаются (happen) have a reflexive -ся ending here?

The verb случаться is used for spontaneous events that occur without a specific agent causing them. In Russian, the reflexive -ся ending is often used to show that an action happens on its own, which is very common for natural events, accidents, or general occurrences.

Why is the imperfective aspect (случаются) used here instead of the perfective?

The imperfective aspect is required because the sentence describes an ongoing, repeated trend. The phrase всё чаще inherently points to a recurring action over time, which always demands the imperfective aspect.