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A Dragon Break occurs whenever weird time shit goes down and Akatosh had to fix it. This usually happens when a mortal ascends to godhood, or a god is killed/split up, or something else involving gods happens. Basically, he picks up all the shards of time and stitches it together as best he can, but there are some minor inconsistencies. Have you ever read Thief of Time? It's a lot like that.
Second, the Nerevarine helped cause the Oblivion Crisis by destroying/banishing the Heart of Lorkhan. The Heart was the key to the Red Tower, one of the metaphysical anchors keeping reality pinned down. It's destruction weakened the barriers between Nirn and Oblivion, meaning that White-Gold tower was the last anchor standing, and it's key being taken out of commission would allow Mehrunes Dagon to invade.
Third, CHIM. To understand CHIM, you need to understand that the Elder Scrolls reality is basically the dream of an immortal sleeping godhead, unawake and unaware. To achieve CHIM is to realize that you are nothing but the figment of a figment of an imagination, while still retaining your sense of individuality. Most people can't do this, and "Zero-Sum", or fade out of reality and become one with the Godhead. Those who retain their sense of self, the ability to say "I", achieve CHIM, and act as lucid dreamers, able to change the world around them as they see fit, and are essentially omnipotent. However, they risk losing their sense of self, and if they do, they Zero-Sum and fade out of existance. It's a fine line, realizing that you are a dream and yet retaining self-awareness. If you forget the former, you go back to sleep. If you forget the latter, you fade away.
I don't even want to go into what Dagoth Ur really was, because that's a whole 'nother can of worms.