What's the difference between 1 and 2? In 1, "did" an auxiliary verb was used for inversion, while in 2, the verb was just inversed. When do you use an auxiliary verb and when do you just inverse it?
1. Never did I dream that he was there.
2. Down came a shower(Not Down did a shower come)
Out went the light(Not Out did the light go)
When the sentence begins with a negative adverbial and the verb is in simple past or simple present, you need to insert do, does, or did.
I never dreamed that he was there. >> Never did I dream that he was there.
Your other two examples do not begin with a negative adverb.
A shower came down > Down came a shower.
The light went out > Out went the light.
The light went out rarely. > Rarely did the light go out.